How can I erase the data from my Macbook without losing my recent OS?

I'm going to sell my Macbook Pro and want to remove all of the data before doing so. I have the OS Lion installed but my machine came with Snow Leopard. How can I remove everything and restore it to basically just the OS without losing Lion? I want to sell the machine with Lion. Thanks.

Apple provides instructions on erasing and reinstalling OS X, so the system will be as it was when it came from the factory: http://support.apple.com/kb/PH4439

These instructions guide you through reformatting the drive and reinstalling OS X via Recovery.

You can reinstall like cablesm suggested and it will have Lion installed but you have to understand that you do NOT sell Lion with it unless you have a bought Lion USB Stick cause Lion is hard bundled with YOUR AppleID and you can not sell it or move it to another AppleID!

Therefore I would sell it with Snow Leopard preinstalled.

Here are instructions copied straight from http://support.apple.com/kb/PH6522

To erase and reinstall Mac OS X Snow Leopard:

• Insert the Mac OS X installation disc into your computer’s optical drive (or an optical drive connected to your computer), and then double-click the Install Mac OS X icon.

• In the Installer, click Utilities, and then click Restart.

• When the Language Chooser appears, select your language, and then click the Continue button (looks like an arrow).

• In the Installer, choose Utilities > Disk Utility.

• Select your startup disk from the list on the left, and then click the Erase tab.

• From the Format pop-up menu, select Mac OS Extended (Journaled), type a name for your disk, and then click Erase.

• After the disk has been erased, choose Disk Utility > Quit Disk Utility, and then follow the instructions in the Mac OS X Installer to reinstall Mac OS X.