My phone battery does not last as long as it once did. Can someone give me some possible explanations?

I'm not sure what kind of phone you have, but if it is a smartphone, then you may need to turn off your GPS locator and your wireless when not using it. Your phone will constantly search for a signal otherwise. You can also adjust your brightness on your display. Having it all the way up drains the battery pretty bad. Lastly, if you use the phone a lot and charge it a lot, it will naturally deplete the battery life. It's also really bad to only charge the battery for short periods of time. It's best to let the charge go through it's full cycle. My daughter wears out batteries constantly because she will put it on charge for 15 minutes at a time. She would be better off if she would just put the damn thing down 15 minutes at a time, but I digress.

Rechargable battaries lose their ability to charge completly the more you use them. They wear out. You will need to get a new battery if you want your battery to last longer than your current one year old battery lasts.

Well, if you leave it charging when it's fully charged, then the battery life will get lower, and even if you didn't, just use causes the batter life to decrease.