The day after the iOS 4 came out I downloaded it on my iPhone 3GS. I really was looking for all the features that the new iPhone 4 has to offer but without the hardware upgrade price. 

Loved how I could organize my Apps but hated my battery life. I blame the multitasking. It's wasn't completely thought out in my opinion. It leaves all the Apps you recently used open forever unless you close them specifically which is a PITA! An option to minimize or quit would be ideal.

So finally I was tired and I wanted my old phone back, I don't need all this fancy features if I don't have a battery to run it! I tried everything I could find on how to revert to my old firmware 3.1.2 and I couldn't do it since Apple no longer signs the old firmware. So I gave up and in the end found something interesting... 

Someone, sorry I don't recall where I read it, mentioned that you should reboot your iPhone to handle this. They basically mentioned this:

1. Charge your phone fully overnight (This give time for the iPhone to index all the files)
2. Unplug your phone
3. Hold the Home and Lock button together for 10 seconds. (This is basically forcing the phone to turns off)
4. Wait 10 seconds
5. Turn the phone on (Leave the Lock button pressed until it turns on)
6. That's it! 

After this I've noticed an improvement on my battery life. Before after closing all Apps before going to bed I'd wake up to 20% less in my battery. Now I've noticed a 5% decrease which is normal for the phone just being on all night. 

And that folks is how you do it!