How do I conserve battery on my Android phone?

I have a Motorola Droid and am looking for good suggestions to help conserve battery power to keep the phone going longer. How do you usually keep your phone lasting all day without limiting yourself too much? What apps drain the battery the most? Is there an app to help conservation?

How can I make my iPhone 4 battery last longer?

I’m the proud owner of an iPhone 4, but as I spend a long time outdoors I used to always carry a spare battery with me. The iPhone’s battery is not replaceable, so I can’t do this. Is there an other option I could use instead of a spare battery??