Any tips on making a cell phone battery last longer, besides replacing it?

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I've had the phone for 9 months and the battery seems to be draining faster and faster. Should I keep it on or off when I'm not using it for a long time? Is there such thing as over charging? How long should I charge the battery? How come it will say that it's fully charged after a half hour of charging, I'll make one 10 min. call and the battery goes down a bar? Does the vibrate ring use more of the battery? The phone company told me to buy a new battery. Of course they say that. That's how they make their money. If I'm doing something wrong I'll just wear out that one too. Any tips?