What is the best way to charge my iPhone 4s?

I usually wait until the battery is under 10% and charge it while I sleep, but I've heard that leaving it in for a while once its fully charged is bad for the battery.
Also, some of my friends just plug it in whenever and take it out whenever.

It's OK to leave it plugged in even after it's fully charged - there's circuitry in the phone to prevent overcharging. Really, any way you charge it is OK - your way or your friend's way, it really doesn't make a lot of difference.

Yes wait until the battery is under 10% but I wouldn't recommend leaving it to charge overnight. It is bad for the battery.

Any way you want. Just be sure to do a full charge at least once a week.