You mention that your service isn't good where you are. The could be part of the problem. It might be using battery constantly looking for a signal. Try turning off roaming??
I know that part of good battery life is "training" your battery. For example, you should completely drain it once a month. If you always charge it when its at say, 30%, then eventually the battery will "settle" at that level and think it is drained at that point. Also, its not good to "overcharge" it by keeping it plugged in way past the 100% charged point (a little while past that is ok since when it first registers fully charged its usually only really 80-90% charged).
Well those are some things that might be helpful!