Another Galaxy S user here.
Battery life is pretty good on this phone for the kind of hardware it has, After I fully charged the phone on last Sunday evening, I forgot to switch off the WiFi and the battery drained to 50% by morning. the remaining battery still got me though the day easily enough. Anyway here are some tips to get the best out of the battery in Galaxy S. I am getting 4~5 days on a single charge for my typical usage which is 1 Hour calls every day and bit of Internet over WiFi and occasional game or vid playback by my curious colleagues checking out the phone. I charged my battery on Monday night and I still have battery left. I am charging it today, but I am pretty sure the remaining battery could have have got me though tomorrow afternoon at least.
1. Use a dark (preferably with a lot of black) wallpaper/live wallpaper. I am using NexusMod Live wallpaper with the dark setting. This save's a lot of battery life on a AMOLED/S-AMOLED screen as compared to something dandy like the Aquarium.
2. Remove home screens that you don't need. Don't add widgets that you don't use. More importantly don't use the Samsung widgets. I have only 3 home screens with the widgets I need.
3. Goto Settings->Wireless and Network->Mobile Networks and enable "Use only 2G Networks" when you are not going to use 3G. Indian users might definitely want to enable this.
4. Install Advanced task killer and kill any apps that you are not going to use regularly.
5. I have left screen brightness on Auto Adjust, but people might want to keep it manually at a low setting.