how many percent of charge are you losing over what amount of time?
if Apple has diagnosed your iPhone and did not replace it, then it is unlikely to have a damaged battery.
So the key to prolonging battery life is finding "secret" power comsuming processes, such as
Power consumption can increase significantly if your iPhone often loses contact with either WLAN, cellular network or both and has to reestablish the data connection.
also it has been suggested here that sending diagnostic data to Apple in the background (you are asked to allow that during the initial setup of your iPhone) has some influence on battery life.
should you use your iPhone mainly for phone calls, have you considered shutting down cellular data entirely or reducing cellular data usage to 2G by disabling 3G connections? 3G (UMTS or HSPA) connections do drain the battery. Also you could limit data usage to WLAN only by disabling cellular data at all. If no data service is needed at all, you could finally disable WLAN as well.
Should Facetime not be used regularily, disabling it can help save some battery power.
Hope this helps,