How do I wipe my MB Air's drive and start fresh?
If you bought a pre-Lion Macbook Air, it should have shipped with a system restore USB drive. This is exactly the scenario for which it exists: restoring your system to factory condition. Back up all your data, so you can keep what you want. Then, plug in the restore drive, and hold down the option key (⌥) when you boot. Select the restore drive, and follow the instructions to install a clean install of Snow Leopard.