Why won't my iPhone charge or turn off?

I have an iPhone 3GS that's about 2 years old. It has been dropped a couple of time, but doesn't have any external damage aside from a couple scratches. When I plug it in, it rarely charges. I've tried a few different chargers with it and it still doesn't charge. Whenever I turn it off, it almost always turns itself back on after a few minutes. Is there anything I can do or should I look into getting a new phone?

Time for a new phone.

Your charger is broken and the reason it won't turn of is because something has happened to it but just let it run out of charge and buy a new charger and take to a shop and ask if there's anything they can do to fix it and if they can then look into a new phone

Instead of sitting here asking people who may or may not know what they are talking about, go and take it into the shop you got it from to be looked at.