Hope the wort chiller posts above make sense. My top tips if i was doing it again would be:
1. the crucial bit of the wort chiller is getting a nice curve without crimping the pipe to get the exit pipe running up the inside of the coil. if i was doing this again i'd make that bend using a pipe bender. I'd feed the pipe bender approx 2 feet onto the coil and make a near 90 degree bend, leaving the pipe bender in place (could be controversial doing this but i think it would stop the pipe crimping. I'd then feed my gas main over the now bent 2ft section after taking a check out of the bottom of the gas main. I'd then start winding the pipe round the gas main (or whatever else you have)working from the ground floor up. This would allow you to wind the coil around the exit pipe without having to worry about trying to squeeze it through the coil after making it. To be honst neither scenario is ideal as this is where you are most likely to bend the pipe. copper is surprisingly pliable until it starts to kink and then it creases very easily, so it was a tetchy moment doing the last section as i almost buggered it. Luckily some gentle work with pliers got it back to normal(almost, my OCD is still smarting )
2. If you can get a bit of gas main thats straight is really good to use, just about the perfect size for a 15-19l pot.
3. give the copper a good heat in the oven, it makes it easier to bend until it cools down, which it does rapidly - an essential property of a wort chiller
4. never straighten a section and then try to rebend it if at all possible, it almost never bends as well 2nd time round. If you follow point 1 above straighten the pipe as mich as you need to but be careful when bending it afterwards.
5. work out the height of your pot and how many coils you'll need before the pipes exit the pot and get a good bend on them to get them out of harms way as much as possible. As others menioned up there if it leaks you are in trouble.
6. the wee mini hose i linked to in one of the posts is the best tenner i spent on my AG setup. does the job brilliantly and i got a free spray gun for my regular hose to boot
7. get your copper from a plumbers merchant and shop around - its probably a tenner cheaper than b and q in most cases. Also check the coil for damage before you take it home/start using it, if a small ding is there it will probably kink when you start bending it.
8. finally, unless its a really serious issue, don't let OCD get the better of you and try to fix it. I did and wasted £16 worth of copper in doing so. In most cases the kink will not be serious enough to severely impair the flow so just let it be
Hopefully this helps, good luck if you try it.
EDIT - crude visual representation if what i mean below. copper is coopper coloured coil, the grey bit is your pipe bender. The yellow thing is a cutout of the gas main and the black blob in the bottom lefthand corner is your cutout for the pipe to come through. The copper would be wound anti clockwise from the ground up.