There are several of these around the net, but most people use an altoids container, a 5v voltage regulator and a 9v battery. I built one of these and it sucked. Because 9v bats only hold around 450mah at best, it charged the phone about 5%, which rendered it useless. It also uses a female USB plug, which means you have to carry around a USB cord with you, which is extra hassle. I wanted to create something that used common easy to find parts, and was cheap to build. This whole project costs around $4 to build, and it charges the phone anywhere from 25-40% depending on the type of batteries you use.
Step 1: Parts list
1x Battery box - $1.99 (I saw it at another Radio Shack for $2.29, but my local one had it for 1.99)
4x AA batteries - $.50 (I went to the dollar store and bought this 8 pack for $1)
Cell phone cord $1.50 (I found one cheap on Amazon, but make sure you get the one that fits your phone)
1A diode (these are very cheap if you buy them in bulk - probably in the 10 cent range).
Soldering iron / solder
Heat shrink tubing, or you could use electrical tape I suppose