I have a Mid 2011 Macbook Air running on Lion. I plan on purchasing a retina pro soon for myself and then handing my Macbook Air to my sister and then she'll hand off her nonretina Pro to my parents. I want to erase all the contents from my Air so that it's as if it's fresh out of the box for her. I don't want to have to upgrade the OS though as I don't want any complications with the laptop slowing down. Is there a way to start fresh and keep Lion?
Go to System Preferences, "Users & Groups," and then create a new user account for your sister. Make sure it's an administrator account.
Now log in with the new account and go to the same feature and use it to delete your account. You will get a dialog box asking if you want to delete your files or keep them. Delete them.
That will more or less remove all your stuff.
You can also go to the Applications folder and delete all the programs you installed, unless you think she might want them.
If you were to sell the computer on eBay or something, potentially to a criminal who wanted to scrape your machine for personal information, then this procedure might not be good enough for you. But since you're just giving the machine to your sister I imagine it's fine.