here's the easiest way to do it and always have it on hand on a usb stick so you don't have to go thru the endless wait of downloading Lion again and again from the net each time you wish to do a fresh install.
you
will need to download Lion once more to achieve this, and you will need to log into the AppStore with your Apple ID - if you don't have one it's a snack to create one and also free.
Lion will cost you a few bucks, but as you already own a Mac that shipped with it, it should be free.
follow the directions from the following link below under:
'Mac Sysadmins are Smarter'
http://goo.gl/yszMx