How should I go about selling my MacBook Pro?

It is a mid 2009 MacBook Pro 13 inch. When I bought it, the machine had a 156GB hard drive and 2GB of memory. I've since upgraded it with a 500GB hard drive and 4GB of memory, the keyboard has been replaced once and the trackpad has been replaced twice. It still works like a charm and I use it for web browsing, Skype, and game development. I would like to upgrade someday soon, however.

What would be the best method to sell my Mac? Should I take it somewhere or sell it online? How much can I get for my machine?