1. Don't leave the flip of your phone open (if you have a flip phone).
2. Be sure to close any applications, games, or web browsers that you're not using at the time.
3. If you are in a roaming area and don't want to receive or make a call for a while, OR if your phone is "looking for service" (has absolutely no signal) and you're not changing locations soon, then turn the phone off or turn on "Airplane mode" in the settings. Airplane mode keeps the phone from making or receiving calls but you can still play games or check your contacts book or calendar.
4. Turn "Bluetooth" completely off if you're not using it. If there is a "B"-looking symbol on your main screen, then it is still on.
5. Set the backlight or main screen to turn off faster. (On most phones, this is under "Settings" then "Display").
6. Buy an extended life battery. They are available for most phones.
...In the long run
1. Only charge the phone when you need to. Most batteries take around 500-1000 charges. If it's on the charger 2-3 times a day, that'll last 1-2 years. (Note: cell phone batteries do not develop a memory like cordless phone batteries do.)
2. Get a service plan with your phone that offers free battery replacements. Most phone batteries get annoying after a year, and most new ones cost $30-$80, so a $55 service plan with two free batteries isn't so bad.
3. Don't spend less than $20 on a charger, car or home. MAKE SURE the charger has an automatic shutoff when the battery is full, or you will quite literally cook your battery (more of a bake than a fry, but you get the point).
4. Don't use "cellboost" chargers (a quick recharger that plugs into your phone). It's like hooking jumper cables to your battery. Just buy a car charger.