1. assault
2. aggressive assault
3. 'Blood for the......Khaela Mensha Khaine?'
4. attack
5. Charge!!!
6. kill
7. destroy
8. demolish
9. and always, always kill as many High Elves as possible ('Burn Uluthan, burn' - battlecry of my Dark Elves force). I cannot help this, but this is the way I see it. I'm aggressive kind of player and I love to play that way. I think there's no point in general tactics, as you can (and should) tailor your army against any opponent individually.

For me there are 3 groups of Dark Elves regiments and characters.
1. Assault: Cold One Knights, Witch Elves, Witch Elves characters and General.
2. Support: Black Ark Corsairs, Crossbowmen, Bolt Throwers, Sorcerers, Assassins
3. Light troops: Scouts, Harpies, Dark Riders. (as you can see I include Scouts that are not released by GW (in the last edition), I use my plastic warriors for that purpose. If you take closer look at Shadow Warriors (those miniatures are horrible), I'm sure you will agree with me that using plastic warriors as Scouts is justified).
I haven't included spearmen, Black Guard, warriors and City Guard because I find them useless. Different story is with Executioners - there are no models available.

And now I tell you how to employ Dark Elves against different opponents:

Against High Elves.

There is only one way to crush High Elves. It always needs bloody melee which is fine for me as it suits Dark Elves Army spirit. I like to behave like Khornate warlord - once you get in Hth you are to win. DE regiments are better fighters and hate HE what makes great combo. But such a plan will only work against inexperienced players. In most cases DE general must outmaneuver opponent and surprise him with troop selection. The battle is won or lost in two first turns almost in all cases. To some extent vital factor is who has first turn.

Troop selection:

1. Assault:

General - although Dark Elf General isn't very potent warrior, but against HE his fighting abilities are much enlarged by hatred, so I equip him for Hth. The army commander is mounted on Cold One and fights with Cold One Knights, so he gets protection of a unit and also stupidity test are taken on his Ld of 10. Special saves are important, because you cannot let your Leader die during first melee. I like most Black Amulet or Golden Crown of Atrazar with combination with Jade Amulet (5pts. save of 2+ against one wound only) to make General even more durable. You can always replace Jade Amulet with something else like Potion of Strength, Amulet of Fire etc. Magic weapon - something cheap but efficient.
Flail of Skulls (30pts. +2S in first round of Hth, 1W=2W) great item because in first round of combat you not only reroll missed to hit rolls, but also General has Strength of 6 and wound almost all enemies on 2+.
Executioner Axe - 50pts, no armour saves, kills opponent outright on a roll of 6 to wound (on a card in WH Magic it's stated that on a roll of six to hit - but we play differently, according to WD), +2S, but you hit last. In that moment armour and special saves become very handy indeed. This Axe is most useful against monsters and tough characters.
Cold One Knights - one regiment 8-12 or even 15 strong equipped with lances but without crossbows. The bigger unit the better.
Standard Bearer is essential and he should have: Standard of Shielding (1+ save - immune to bowfire and attacks in Hth of S3),
Banner of Might (not only your knights will hit on 2+, but also Cold Ones hit on 3+ with their strength of 4 and 2 attacks it is great help - assuming you fight troops of WS4),
Battle Banner (no need to explain) or,
War Banner - +1 to combat resolution, like additional row of Knights.
Also remember that your Cold Ones cause fear so make your opponent take all required tests.
If I include Champion he will have something against High Magic or shooting, be it Amulet of Fire, Spellshield or Ruby Chalice (think hard if you want to include this item, because it activates after first casualties. So salvo of 2 RBTs that are in battery would demolish Cold Ones and only after taking those casualties they will be -2 to hit from shooting).
In larger battles Battle Standard Bearer is a very good buy for this regiment, not only giving extra fighting power but also second magic banner. Consider something cheap like Standard of Shielding or War Banner, than you can give the Regimental Standard Bearer Battle Banner and suddenly you become invincible.
As for me Cold One Knight Regiment is right place for General - that's why Blood Banner is useless - you test stupidity on 10.

Witch Elves - one or two regiments each 20-25 strong with Standard Bearer and sometimes Champion.
Favorite banners:
Banner of Might - very handy when facing White Lions, it somehow negates that special attack that grant your Regiment with -1 to hit and also increase probability of killing more High Elves with +1 to hit modifier in all other cases.
War Banner - cheap banner, +1 to combat resolution.
Dread Banner- you cause fear, so if WE outnumbers opponent and win combat resolution the foe is automatically broken. WE Regiments tend to be rather large and one has fair chances to destroy whole regiments. Last but not least regiment you are charging may fail fear test - what makes combat very one sided indeed and in your favor.
I never give magic items for my witch champions, because I can't see the point in it. They have S5, 7A (with Cauldron) and only 1W and T3, so you need very expensive magic item with special save to save them from death (and with Cauldron they have unmodified save on 6). With such line of stats they are combat monsters and I don't think they need that Ogre Blade for 40pts. and only +1S, as magical weapon cannot be poisoned. The one and only magic item they may carry is Ruby Chalice if you don't have free slot in other characters or maybe Potion of Strength.
To further boost my army offensive potential I include Witch Elf Heroin in each regiment. As it always pays to protect your investment, they will be given Black Amulet, Golden Crown of Atrazar, Armour of Fortune or Armour of Meteoric Iron and Dawn Stone (25pts. - reroll failed armour saves) - for 50pts you have very effective defensive combination of magic items.
I rarely give my Heroines magic weapon, Witch Elf Heroin with halberd and with presence of Cauldron will have S6 9A (against basic RnF and with Banner of Might you will hit on 2+, wound on 2+ with -3 save modifier). HE have rather low armour saves so -3 modifier seems OK. You can always give your heroin not only halberd but also double - handed sword and additional weapon so she become very versatile for 5pts.
To strengthen your regiment you may include Assasins in your regiments. One may have Black Gem of Gnar if you are facing White Lions with General or flying monsters. When you use Black Gem against General, White Lions will not be immune to psychology and you should brake them and kill in pursuit. That monster charging from flank won't take your rank bonus when you gem it.
And of course you need presence of Cauldron of Blood, no need to explain.

2. Support: Sorcerers - I used to include two of them so you can retain two cards from one magic phase to another. One level 3 with Skull Staff which is necessary as you start with 5+ dispel against any High Elf Sorcerer whatever his level is. Other useful items: Potion of Knowledge will help you in magic phase as you have to only reinforce your spells what will draw away your opponent's wind of magic. Possibility of becoming stupid is rather small so this risk is acceptable. Book of Ashur is of dual use: first you have access to Chaos (Tzeench or Slaanesh) deck that is very effective and costs less in comparison to Dark Magic. I personally think that even with Book of Ashur you should not use specific race's spell decks. The exception I made for Chaos is somehow justified as Dark Magic has its origins in Chaos itself. Second thing about this item is that that it raises your sorcerer level and it's less costly than fielding level 4 magician. Cloak of Mists & Shadows - you will need this to transport wizard close to enemy as your spells are of rather short range and require line of sight. Ring of Corin - together with Skull Staff great against low - points items like Black Gem of Gnar, Skull Staff etc. This wizard stands proudly in Black Ark Corsairs Regiment with heavy magic support from Banner of Sorcery (if you don't roll 1 of course) or roam on battlefield using Cloak of Mists & Shadows. The second sorcerer is level 1 or 2 and has dispel scroll(-s). He will be included in Witch Elves Regiment (I advise to use his/her dispels fast, as magicians are prone of dying when melee starts - WE are strictly Hth unit) or Black Ark Corsairs if your higher level wizard has Cloak of M&S. Black Ark Corsairs - right place for your sorcerer. Unit size vary from 10 to 16 or even 20 (they are armed with additional weapon and crossbows - but if I need some extra points crossbows are first to go) including Standard Bearer with: Banner of Sorcery - if you want shooting/magic based army it's the best choice. Standard of Shielding - if you expect lots of archers. Banner of Defiance - if you expect that you'll be charged or want have greater combat resolution. When playing HE the note on the card 'never pursue' does not take place. You should roll a dice and on a roll of 4+ you can pursue - this is all connected with hatred. Look for 'Irresistible and Immovable' in WH Magic. All in all you start with +7 combat resolution. Try include assassin that will protect your magician (Black Gem of Gnar). Corsairs also let you field two RBTs. It's great disturbance for HE player as he is to have armored cavalry or foot troops with lots of ranks. Entertaining option is to field at least 16 strong regiment with General riding Cold One, sorcerer and assassin. Hth power lies in hands of your general so equip him accordingly. If you want use Corsairs offensively include WE Heroin - but beware of frenzy. Crossbowmen - if you want to play defensively field one unit of 10, just to have those RBTs. They are not so bad, players are exaggerating about their weaknesses. I agree that light armour they have to wear is waste of points that could be used more effectively elsewhere. Deploy them in one line, so all can shoot. RBTs - to shock your opponent use 4 of them in a battery with your General who should have Amulet of Ravensdark to drive off monsters (eagles especially). In 2000pts battles 2 of them is enough -spread them along your battleline and use them to disturb HE and turn their attention and shooting power against RBTs, while your assault force will charge main enemy force unscathed. 3. Light troops. Dark Riders - very recommendable, unit size 5-7, as you must somehow deal with those RBTs and Chariots or slow down Silver Helms or Dragon Princes. If you have enough points try to include Champion and Standard Bearer. Give Champion cheap item - Blade of Leaping Copper (+1A) is good enough. Harpies - unit size 5-16. Same task as Dark Riders. Scouts - one or two units some 7 strong. General task: disturb HE, destroy chariots and RBTs, or die trying. Remember that you hate High Elves, so you can bog down entire unit of shock cavalry just charging them (no bonus for lances). If they won't kill you right at the moment, you will easily pass break test on 10.