Several things could be causing this. For example:
1.) The battery being too old. (These things do have a shelf life - ~2-3 years.)
2.) When you first got the phone, you may have had the phone plugged too frequently. Your exuberance can shorten the life expectancy of the battery. (Understandably, you don't want to run out of power when you need it, but you need to give the phone charging a break.)
3.) You may have applications that needlessly run all the time. Some apps continually ping servers for the latest information / updates / messages / location / etc. Take a look at those unnecessary programs and either shut them down or control them. If you did find some apps that could be a problem, determine whether you need to be updated all the time even while you're asleep. If you've found some of those apps, then I would check your wireless phone bill to see if your "data usage" is running high. I have found that those offending apps might be pickpocketing you. On the upside, your wireless carrier is always willing to sell you more "data". ;-)
For example, any application that requires your phone's "Location" setting to be set to "on." (e.g. Facebook, MapQuest, GoogleMaps, etc.)
4.) I am sure there are more...
I hope this helps.