Mine's still pretty strong, going into the second week of use. The more you use it obviously the faster it will drain.
I also don't charge mine overnight. I charge it when I come home from work, unplugging it when it hits 100%, use it a bit, and then charge it when I shower the following morning which usually gets it back in the mid 90's.
Apple's iPhone user guide also suggests that you run the battery to 0% once a month. This will help recalibrate the percentage meter.
It also mentions that it's a smart system of charging.
Your phone will charge from 0% to 80% in about 2 hours, and then take another 2 hours from 80% to 100%. So, if you just keep topping it off the percentage might be misleading (is it actually powering off from low battery, or just giving you the warning?)...
I'd also make sure you close apps and see if that's any better. Double tap the home button so the bottom bar comes up, then push and hold one of the icons until they wiggle. Push the red minus to close all open apps. Don't worry about closing Messages or Phone, everything still works, you'll still get texts and e-mails and phone calls.
Also, as was suggested above, if you're using apps and you see the little compass arrow appear in the top right, your phone is using extra power to give your location to each app. This can be customized in settings. Disabling location services, bluetooth, wifi when not needed can improve your battery life.
Also, make sure you use an auto-brightness. If you have it set to auto-lock when idle, this will help improve battery life too.
