How to prolong the life of lithium ion batteries?

hi every one. Can some one please tell me how to prolong the life of a lithium ion battery in a new mobile phone. (my new iphone 3GS) what what are some of the best ways to take care of them to make them last a long time and what are the best ways to chrage them. ie do you discharge them fully then recharge them or do u charge it at half cappacity? and if you have any tips that will ensure it will have a long life expectancy please let me know.