1. Get a full night's sleep so you go into the store refreshed and clear headed. The last thing you want is to be tired and not paying complete attention. You're going to be making quite a purchase. You need focus.
2. Have a filling (but not heavy) breakfast that morning so you are content but not lethargic. You don't want to go to the store hungry as that can be distracting and give you the feeling that you want to get out of the store, quickly, so you can go eat. A heavy breakfast can induce a drowsy feeling and an almost "indifferent" attitude. You want to be engaged and assertive, not indifferent.
3. To warm you up, watch at least a half hour worth of tv (but not more than 2), before going to the store. This would help ease any nervousness, anxiousness and uneasiness that you may be feeling. You don't want to be on edge when you get to the store. Instead, calm, relaxed and ready to go. You may want to avoid any super stimulating activities like playing videogames before going to the store. Those kind of things may wind you up too much.
4. When you get to the store, browse and play around with things. Don't go right for the MacBook and a sales person. Look at the various computers and accessories and then (after 5 - 10 minutes or so) make your way to the MacBook. This would ease you into the purchase and allow you to be fully aware of your surroundings.
5. Hopefully, you've done your research by now. You'll be getting a great deal. So, don't expect any extra freebees. Apple is not known for haggling. You'll likely just get a discount on MobileMe.
Also, don't forget a camera! You may want to grab a picture of you buying your new Mac!
...And welcome to the Apple family!
