1. reread books 1-5- yes, I know most everyone has already thought of/done this, but it never hurts to suggest it.
2. find/plan a release party- there are loads of midnight release parties you can attend. Potterparties.com has a comprehensive listing of them, and you can search for your area. If there isn't a party in your area, have your own! For OoP, my friends Jordan, Keyla, and my sister went to our Barnes and Noble until 11:30, then came back to my house for a minimal amount of sleep (Jordan plugged her years and talked while Keyla and I watched JKR's Dateline interview) and the following day, read OoP by Jordan's pool. Your party doesn't have to be extravigant, just fun!
3. visit HP sites- Mugglenet and The Leaky Cauldron are great sites for the most up-to-date HP news. Message boards are great too becasue one can share their fanaticism with other fanatics!
4. lock yourself in an empty room so you will not be tempted to kill yourself or others in agony of waiting. (hehe just kidding).
No matter how you spend these last few days, we all know it will have been worth it when we, along with Potterheads everywhere open our books and discover that the Half-Blood Prince is none other than...