Check for running apps, and turn off those that you don't need running in the background.
Look at settings>about phone>battery status then click on the battery graph and look at the color of the greenish bar under it. The darker green the better, the more yellow-brownish the worse for your battery. A prl swap helped me with battery drain issues due to poor reception at home (but it didn't increase speeds), tower hopping can be a huge power drain.
Set your screen timeout to 30 secs or less, and your screen turn off to immediately.
Be sure widgets don't update too often, do you really need to know the exact temperature every 5 minutes? And don't let your accounts sync too often, better to tap on the email or sms icon when you have a moment to check them than to have them syncing constantly even when you don't have time to read them.
Of course, keep your screen brightness as dim as possible, this holds true on any phone.
WiFi definitely drains the battery more than 3g, but poor phone signal is the single biggest battery issue for me. Set your wifi supplicant scan interval to 300 or higher (I use 500), this means it will only scan once every 300 seconds or whatever you set it to. The only issue this may cause is automatically finding wifi networks, it could take as much as 2-3x as long to locate the wifi network automatically. Depending on your sync needs, you may want to set wifi to turn off when the screen turns off as well (I believe that's the default setting, under wifi settings, press menu key then advanced). No issues when turning wifi on/off manually.
Read mmarz's tweaks thread for more battery saving tricks.