What is the best way to wipe your MacBookPro, Mountain Lion, and start again.

My MacBookPro 13" i7 2010, is pretty much full up and is becoming quite sluggish. I have recently put new memory in and increased it to 8G, which has improved it a little but it is still not as fast as it used to be. What is the best way to wipe it clean and start again? Everything is backed up on my Time Machine external HD. Please help.