And what I did:
Firstly, cut polystyrene into hull shape, i recommend you use something sharp and long and definately not scissors. cut out a whole somewhere in the middle to fit the poster tube snugly.
Then cut bin bag into a few strips a few inches wide, and tape them together to form a plastic loop about 1 1/2 times bigger than your hull (this might be too big...) Now you stick one edge of the loop to the top edge of the hull starting with the middle of the two longest sides, and the corners, and then pleating along the edges to remove the slack.
Attatch teather lines (fishing line in my case, but string or tape would work too) to the bottom of the inside of the skirt, in the same places you started sticking the top edge down and maybe some more if it all flaps around when you power it up. Stick the teather lines to the underside of the hull so they are all the same length.
Thats the Mk1 body finished.
My first fan assembly was made by cutting two notches opposite each other around the hole in a cd, which the screws that held the motor went through, then cut a section of poster tube (2 1/2" size is a perfect fit for most hairdryer fans) and cut a few big slits an inch or two long into the top. Screw the motor onto the cd, and center and glue the tube to the cd. Done. Bung this in your hole, attatch it to a suitable power supply and it should lift up. it may look a little, or less, like this:

Please excuse the horrible carpet! I have no choice in the matter.My room is also only slightly bigger than the picture... My one of these had a tendency to jump up and down at the back, and only inflate on one side.
It is pictured using an old scalextric transformer as a power source, which is supposed to give 12V, but due to years of use as a bench power supply, and deodorant ignitor now gives an amazing 20.6V for some reason. I keep forgetting this, and many a transistor has been blown due to my forgetfulness.

This used:
More Polystyrene
More bin bag(s)
Very sticky masking tape
very crappy masking tape (w h smith)
glue gun
Fan assembly Mk2 (see below)
And was made by:
Making a body the same as above, but about twice as big, and another sub-hull the same shape and size as #1's hull, but with no hole.
use some polystyrene blocks and glue to stick the sub-hull to the main body, leaving a gap in between.
Cut the closed end off the bag, then fold it in half or thirds. Cut the folds so you have two wide loops, then open and stick them together to make another big'un. Adjust the size for the size of your hull.
Attatch the loop to the hull as above, then do the same underneath on the sub-hull-bit. you should now have a thing that has a big sheet of polystyrene on one side, and a small one on the other, with a a black plastic skirt around it.(if you don't, something has gone seriously wrong...)
now stick some holes in the sub-hull with a pencil or something, or cut a few bigger holes, but not so many that the skirt wont inflate.
jam in your lift fan, and watch it rise.
I somewhat improved my fan system for this one, by getting a new piece of tube, and glueing one of those lego sticks with the holes in across the top, and putting the motor screws through the holes. This increased the airflow by at least a quater.

My one of these could lift a large hammer and a magnetron (I had one lying around in my room) at the same time, but didnt hover very well, probably because it didnt have enough air escaping through the bottom.
Here is #1 and #2 without lift fan, you can see how #1 has 1 hull and #2 has 2.

This was adapted/improved from an apparently well known circular design used in schools etc that used a vacuum cleaner in reverse. So this design will work if you want to scale it to fit adults, children, dogs, hamsters (not recomended unless you like chopped hamsters, but you could use a baloon or something for air)

It uses:
1 large cardboard box
1 bin bag
1 bolt (about 1" i think)
1 washer (no sharp edges)
1 friction nut (the ones with the blue plastic bit inside)
1 lift fan Mk2 (see above)
1 hairdryer fan/heater assembly
1 radio reciever + servo system
1 relay
Sticky tape
Wire
Black marker pen
(Ballast objects might be needed for balance)

How to make it:
Cut two large circles out of cardboard. Cut a hole in one that is big enough for the lift fan tube to fit through, this is the top, and cut one in the other piece that is slightly smaller than the inside of the tube, this is the bottom.
Cut off the closed end of a bag, and cut down the side to make one big sheet of plastic. Place a circle of cardboard on the plastic sheet and draw a circle about 3" larger than the card. Mark the centre, and cut out the plastic circle.
Draw 6 equal (60 degree) segments onto the plastic and cut a small hole at about 1/3 of the radius on each line.
Cut a hole in the centre of the plastic and bottom card circles put the bolt through both, using the washer on the plastic side so it wont rip, and screw on the friction nut untill it will hold itself still. Then tape the edge of the plastic to the card, pleating it as you go round
(to get this even, draw a circle an inch from the edge if the card, and tape the plastic so the curve of the plastic is exactly opposite to the curve of the line, youll see what I mean when you try it)
You should now have something like this:

And another card circle similar to the one pictured.



Now it really depends on what you have availiable in the way of control systems, but here's what I did to give you some ideas:
For my thrust fan I used the fan/heater assembly from a hairdryer. First I took it all apart and removed the heating element and all that stuff then put it back together again, which left me with a fan in a plastic frame with a heat resistant cone (hopefully not containing asbestos!) coming out the back, this was perfect for a directed air stream.
I mounted this on two cardboard supports which I glued to the main circle.
I then built a twin rudder system out of cardboard and lego (it has no end of uses) which I mounted through the card disc. The radio and steering servo came from a cheap remote control car, which used some really wierd connections (+3v, +6v, 0v, three drive motor connections), so i had to make a custom power supply for it and work out what the hell all the wires did. The problem with it was that the drive connection only gave 5v, which wouldn't run the thrust fan, so I tried using various bits of microwave to make a relay set-up, but all the relays in said microwave oven where 18v, so no luck.
The only other relays I had where Big Mama 1x1x2" high current relays I swiped from an old Terretorial Army base where me and the rest of the Fleet bmx crew built a skatepark. They used 24v, but they also had see-through covers which came off allowing me to take out the return spring and stretch it so it worked on 5v.Problem solved.
So I ran the thrust fan off the relay and separate batteries. The lift fan was controlled simply via the main on/off switch.
I used the steering motor and gear system as it was, glued it to the underside of the card, and attatched it to the rudder mechanism using a piece of ring main cable as a control rod.
I don't know what voltage the fans are supposed to run on (probably 12v) but they seem quite happy at 27v (but for how long??) which is the least at which I could get enough thrust to go anywhere at a reasonable speed.
I attatched the (10!) batteries and the radio using ring main cable (another thing with more uses than you can think of), and then attatched the two card discs together using pieces of poster tube as spacers. Finally I adjusted the hole size under the skirt 'till it glided across the carpet when pushed Here is the finished article in all its hoverational glory: (i had to use that word somewhere)

the problem with this one is that it is tempremental and rides too high, so it tends to dig in at the front when thrusting because its c of g is too high up (I guess) or doesnt go anywhere.
This could be sorted by doing up the nut all the way and widening the air holes in the skirt to provide a greater external air cusion.
It also spins round slowly because of the gyration effect (or whatever its called) of the lift fan motor.

*update*

Ihave now fixed the crappiness of #3 by sticking all electricals etc on top instead, so it sits properly on the skirt. Its pretty crazy, especially when used on lino: it goes too fast, turns nicely, but has no reverse, so you cant stop!
These pictures were taken just before major testing in my kitchen, when the batteries "got a bit toasty" and melted a couple of the wires...:

If being grounded has taught me anything, it is that there is way too much stupid stuff in my room.