Are you in an area with a poor signal? If so, that can be a MAJOR drain on your battery, even when the phone isn't in use.
Before I moved into my new house, I was able to talk on the phone for a few house, send lots of texts, use the web, facebook, twitter, etc. all day long, and I wouldn't need to plug the phone in until I went to bed. Since moving, my battery can easily go from about 50% to 10% (where it starts beeping and flashing the green LED) in three to four hours without even being touched. I've changed a lot of settings and removed a bunch of apps, but I still have to charge my phone or switch out the battery midday.
There are so many factors that play into battery life, so if you're having problems, it may take a while to figure out the exact cause. :\