After using my iPhone 3G for years I gave it to my brother who also used it quite a while. But since he loves all the modifications possible with a jailbreak he soon noticed that the battery keeps draining faster every day, so he asked me to help him out.
I ordered the 3G Replacement Battery on iFixit and followed the guide on this site. Everything was explained perfectly, so I didn't encounter any errors. I also used the opportunity to clean the inside of the 3G... that was a lot of dust and stuff =D
One thing that I noticed while cleaning - take care with the rubber thingie around the display, due to it's age mine was kinda weak and was ripped at one end (nothing a bit of glue couldn't fix tho).