My tests in the past indicate that merely putting the EE to sleep (by closing it) gives a "standby" battery time of 16-20 hours. This effectively gives you all day usage. It also helps to turn the wifi off when you're not using it (most conveniently by installing the Power Control widget to your home screen, for example), as wifi draws almost as much power as the LCD.

If you're trying to stretch battery life out to 2-3 days, well...