If you happen to have an RTL 8188fu - I found this set of instructions REALLY handy:
http://www.rajib.com/?p=1680&i=1
Essentially you can download the drivers from here: http://downloads.fars-robotics.net/wifi ... u-drivers/
Make sure that you match the driver / kernel version with the Pi you are installing, extract the file, run the included script and reboot - pretty simple.
Note: Found this while helping Andy G6ULX with his WiFi on a Pi B (the original one).