How to prepare a frame for a new paintjob
A few tips:


Painters don't know anything about bikes. If the paint shop is not specifically a bicycle paint shop, they may still offer to strip your bike of components for you. Don't let this happen! Most painters are unfamiliar with the intricacies of bicycle mechanics, and may improperly remove or damage some components, or not know or have the tools to remove others. For example, I've seen the pressed-in cups of the headset get painted over on more than one occasion.
Remove all the components. If you're not comfortable doing this yourself, a bike shop will be able to correctly do it for you.
Make sure the painters are aware of what needs to be masked off. Areas with exposed threads such as the bottom bracket, water bottle cages, braze-ons for racks, fenders, or brakes should not be painted over! If the paint goes on thick, you might not be able to use these threads without re-tapping them.
Don't worry about frame prep. This is what the painters do know how to do. They'll know the right methods for removing the old paint, such as sandblasting.
If your frame is being sandblasted, worry a little about frame preparation. Figure out what quality tubing your bicycle frame is made of. If it's heavy and thick, don't worry about it. However, if it's a nice, lightweight tubing such as most Reynolds brands, the tubing is in fact very thin at some points, and should not be roughly sandblasted! If the 'sand' is not fine enough, it may dent or severely weaken your frame.