From the beginning you should ask your phone directly why it is loosing battery charge so fast and it will answer. To do so you should find the “battery usage” parameter in the phone's settings. After that you simply kill (disable) everything you do not intend to use.
Your phone's screen is one of it's hungriest parts. It uses a tremendous amount of the phone's charge. But even here you can do something. First of all – decrease the brightness. That way you will save some charge for a few more calls or fun gaming moments. You should also have a dark background. Android devices use less charge to reproduce black color. This is also a great tip for people who are doing mobile software development. Try giving your apps a darker background and it will be less energy consuming then software done by other app development teams. You may even use the “bGoogle.com” link. This way all of the Google search results will be displayed on a black background.
Don't forget to turn off the Wi-Fi and Bluetooth networks when you are not using them. It will save you a lot of the batteries charge, trust me.
Also, turn the GPS module off, since when it is on it is constantly using various resources for satellite connections. Try to use the navigation abilities, provided by your cell network operator.
You also can turn the vibro off if you don't need it since, like the GPS, it is also constantly using the battery charge.
Try using only the second generation networks. You can do so from the phone's settings. Just tick the “use only 2G networks”. You should especially do so if the area you are living in does not cover the 3G and 4G networks at all. You can always have all the connections you need via Wi-Fi if necessary.
And, of course, don't forget to carry the charger. You never know when you need one.