1. Backup your important files.
Before you prepare to send your old laptop off to a new home, make sure you've made backup copies of everything you want to keep on an external hard drive. That includes pictures, music or anything else that is difficult to replace.
2. Deauthorize your software licenses.
If you plan to re-install any of your software on your new machine, make sure you deauthorize your old computer. Some software is savvy enough that it will prevent you from installing a second copy until the first has been unlicensed.
3. Wipe the hard drive.
Don't just empty the trash. It won't make your sensitive info disappear. Instead, make use of free or paid programs (like Darik's Boot and Nuke or Drive Scrubber) that can help you completely wipe the disc. Plus both Lifehacker and PC World have guides to help you.
(Top MacBook Image: Quick and to the Pointless)