How do you find a great "bank" or target for a land grab? Here's some techniques to use:
Crystal Eye: Crystal Eye a kingdom and look for targets that haven't attacked in the last two days and that look inactive. One's that are particularly small at whatever point in the game your in may be banks. Early in the game it may be provinces with 180 acres and 12k NW. Later it may be provinces with 400 acres and 50kNW. These are great possible targets to crystal ball to see how much gold they have.
Crystal Eye can also tell you if someone just sent out a sizable force on an attack. If you find someone with a suitable amount of land and at least 25% of your NW who just sent out a large force they may be a great target to spy military and try to hit.
The Over Run: Just finding a small target and that has realatively high land for it's NW, checking it with crystal ball and/or spy military and taking it's land. Early in the game this is the way to go. Land farms, inactives and semi-actives abound. These are great targets to hit 4 times and then message their regent letting them know you were hitting an inactive, land farm ,etc. These targets usually have a lot of GC's early too. These can be good targets for theft as well. You can spend the whole game doing this and hit a million NW and 3500 acres with your free land and exploration thrown in. At a certain point however, this definitely becomes newbie bashing and you will also fail to grow optimally unless you move on to bigger targets.
The Power Grab: Here's where the rubber meets the road for the top players. Find a target at least 25% of your NW and with weak NW for for his size. Preferred targets are in this order: Halflings, Humans, Dwarves, (Because they may send out too many of their elites), Orcs and Avians (because they tend to have weak D because of their reliance on specialists), then Elves and last Faeries. Look for targets that can't hit you back and have no one on their kingdom that can hit you back. You don't want to lose all that land you've just gained. Once you find a good target you can start with a crystal eye or check your notes (next technique) to see if they've attacked. If they have troops out (for all except Elves and Faeries) then do a Spy Miltary. Ideally, then you will do a survey to check for Guardstations. Once you see the provinces defense send 125%-150% of their defense to make it likely you will get through. 150% should do it 95% of the time. Once I successfully grab I usually send a message saying something like " My attack on you was just a land grab. As long as I and my kingdom are not attacked, thieved, or cast on I will not attack again." That is usually successful in discouraging attacks. This is the key to becoming a top player. Late in the game you will also find lots of great goldmines just looking for land land grabs. Hitting big targets, finding these great goldmines, and gaining large amounts of land without getting hit back is the key to being a top player.
Taking Notes: This is a huge factor that will save you a lot of time and will really help you become a top player. If you find a target that you can almost hit but not quite, could hit if they sent their troops out, find a great goldmine, looks inactive but doesn't have quite enough gold yet, etc, then write it down and come back to it. Write down the fame, NW, and amount of land they have. If you come back a few days later and it's unchanged then you may have a nice goldmine to work. If it's just changed they may have sent their troops out and may be a nice target. Nothing could be nicer than logging on and having a nice target ready to grab.
Final Thoughts: Having great thieves and wizards is a necessity. You should have at least 10% guilds and a great thief ratio. This is crucial in order to get through on Crystal Balls and Spy Miliatary ops. Being diplomatic goes a long way in making sure there are no retaliations. Also, don't leave yourself open to get a grab. They may have friends and the next thing you know you may get hit 5 or 6 times if you leave minimal defense in house. Keep all these factors in mind and you should be able to get all the land you need.