If you're expecting a call and your battery icon starts blinking, the first thing you should do is find a charger. But if that's not an option, here are ten things you can do to hang on. We'll skip the usual tips about the 'memory effect' and which battery type is better — NiCd or Li-Ion — because you don't really have a choice in that.
Close Background Applications
If you're using a Smartphone, close applications that you don't need. Applications that stay active in the background use up a bit of CPU, which uses up battery. Make sure you 'exit' the applications from the menu, not by pressing the 'End' key, as that merely puts the application in the background. In Series 60 Smartphones (mostly Nokias), hold down the 'Menu' key to get a list of all applications running in the background to close them. In Windows Mobile 5 phones, open the 'Memory' application and check the 'Running programs' tab to close them.
Turn Down Screen Brightness
The screen is one of the most power-consuming parts of the mobile phone. The lower the brightness, the lesser power it needs. Keep it as low as you can, so long as you can still see it! Also, some phones like the E61 and the BlackBerry Pearl come with auto-adjust features that increase the brightness in brightly lit areas and dim it in low-light areas. If you're setting the brightness low, make sure that auto-adjust is turned off.
Don't Use Animated Wallpapers or Screensavers
The animations in wallpapers and screensavers can drain a bit of battery as they also consume a wee bit of CPU. Turn them off.
Turn Off Keypad Lights
If you've been using the phone for a while, you may be able to use the keys without seeing them. If this is the case, the keypad backlight can be turned off. However, not a lot of phones support this, but it's worth considering if your phone does.
Decrease Screen Standby Time Out
The time till your screen dims out and/or gets turned off is configurable in most phones. 10 seconds is good, 5 seconds is better. Set it as low as you can go without it becoming inconvenient.
Very helpful and effective tips which has increased my cells battery life and offcourse communications for longer minutes. 
