What is the best way to save iPhone battery?

Since I've moved to iPhone 4 from a 3Gs, my battery drops down extremely fast. If i use it for 20 minutes at the beginning of the day it goes from 100% to 60% and by 3PM with normal use it's at 20%.
What are the best settings to have as little battery drain while the screen is sleeping while keeping the internet and cellular connections active?