3GS Battery issues

Okay, the battery for my 3gs phone improved slightly when I did a hard reset and restored it earlier, but it only last like 2 days.Now the battery gone bad again for instance losing 20%+ when I leave it overnight.I've tried to do hard reset on it for a few times but the time since last full charge for usage and standby(in settings->general) are both still the same (e.g.: usage 7 Hours,54mins ; Standby 7 Hours,54mins).Please tell me what to do cuz I need to use my phone often.Thanks.

??? - I have never had any smart phone last longer than about 2 days.
If that's an issue, reckon I've had an issue with the battery on every smart phone I've ever owned.

Maybe if you turn off wifi, bluetooth and every other thing running - all notifications, location services, auto mail downloading, etc.

Of course, what would be the purpose of a smart phone then?

I'd personally recommend you charge it every other day.

To add to what bobtomay said also do smaller recharges regularly and you battery health will stay in good condition.