There are cheap and expensive options. If you need long range or painless operation, buy a commercial system. If you just have a basic requirement and/or funds are low, then you might find this a better solution as it's a home-brew wifi cantenna microwave waveguide antenna that costs around Ł5:
http://www.pelaginox.com/pc/pc-cantenna.html

I'm a radio guy so the theory, formulae and practical stuff are all pukka. I get half a mile range with this cantenna on my boat, just to the normal sort of base units owned by unknowing but cooperative DSL subscribers. You can get double the gain if you want to try harder, with a bi-quad woktenna... And for 3 times the gain, get an old satellite dish and change the little active element in the centre. Heaps of fun for very little cash.

chris