I've used many li-on and li-po batteries in various devices. Even though this information is for using li-pos in R/C vehicles, the same general principles apply that have been mentioned in previous posts by PPCGeeks members.
http://www.rcgroups.com/forums/showthread.php?t=209187
I've personally had a 2s li-po battery explode into a flaming ball of fire because the manufacturer labeled the capacity wrong and I charged it more than 1C. The battery was puffed up and hissing. I knew what was going to happen and luckily it exploded when I was opening the outside door of my house instead of while I was in my room. It can seriously burn a house or car down. I also realized that when a cell goes below ~3.2V in a battery pack, it ruins it. This happened to a 3s4p pack I was using even though I balanced it while charging, the setting on my voltage cutoff got messed up. When I recharged the battery, that particular cell was bloated and wouldn't balance out. Granted, some of this doesn't apply to a cell phone but it shows how serious it is to take care of your li-on battery.
Here is what I usually do with my batteries:
1. I make sure they're fully charged when I received them.
2. I never deep cycle a lithium battery. This is a no no. They aren't ni-cd or ni-mh batteries. If you've ever used lithium batteries on R/C vehicles, you know that you absolutely must have the proper charger which in this case, is supplied with our phones. For charging my R/C batteries, there isn't a cycle option on the charger for lithium batteries; it's only on ni-cd and ni-mh battery options.
3. I find it best to charge the battery whenever possible but not for a brief period because it'll get hotter when doing a short charge burst, then using the device. Heat is an enemy of lithium batteries. Don't leave it sitting in a hot area or it will catch on fire.
4. If you're going to store a battery, leave it at a 40% charge. You can stick it in the fridge but don't put it in the freezer!
Even though some of the information is repitive and the article applies more to R/C batteries than cell phone batteries, the general idea is the same. I'm glad that our cell phone batteries are safer and more forgiving than R/C li-pos because I don't want to charge my cell phone in a fire suppresent bag so that in case of a fire while charging li-pos, it doesn't catch everthing else around it on fire because of getting too hot. It's really easy to take care of lithium batteries and they're not any more dangerous than ni-cd or ni-mh batteries when you take care of them properly. It's when you don't know what you're doing when you can run into disaster.