Good stuff already posted, just wanted to add that a 12 tooth is a good mod for the sand if you've got some sand tires that bite like a set of haulers or extremes. If you're using a cheap set of paddles (like a gecko) or stock tires then you'll want the taller gearing.
Sorry to but in but Mike I have a set of Sandstars when i go to the dunes about 6 times a year and was wondering if I go with a 14T would it be good? it feels like I am spinning the tire on the bed. i have had my shop break them down and re-do it but still feels like they are spinning.
What kind of pressure are you running? I wouldn't really go lower than 3 pounds on a molded tire simply because they won't give as much and that can make spinning on the rim more likely. You may be able to remedy the problem by either using 2nd gear to start (if you're not already) or just by bumping your tire pressure up a pound or two. They'll spin more in the sand but they'll be less likely to spin on the rim because of the increased pressure being exerted on the bead to keep it in place.
To know whether you're spinning on the bead you need to mark the tire and wheel in the same location before a ride. Then check when you're done and see if the marks still line up.
Hmm, odd that they'd spin that much. Aside from a bead sealant, bead locks or running screws through the bead I'm not sure I've got much of a solution for you. A guy I ride with has 20" SS's on 8" douglas rims and hasn't had that problem, nor have we seen it on a banshee that runs them.
If you're running a 12 I'd go to a 13, but if you go to a 14 you're going to lose the ability to start in 2nd. It'd be worth a try and should lessen the strain on the wheel because of the lower ratio but that also means the tire won't be gripping as hard.
Sprocket choice depends a lot on the terrain but I agree that a 12 tooth can make shifting too fast for some conditions. The location of the powerband and suspension/tire setup has a lot to do with it as well of course.
Sorry I can't be of more help, but I would certainly try the 14 out and see if it helps the problem.
From what I've seen a 12T is quicker off the line but after that the 13T or 14T actually has more acceleration after that. Seat of the pants says the 12T is quicker but when you put the bike up against another one and try both we have found out that its only the initial quickness that has improved. The bigger sprockets utilize the engines torque a little better and let it work to accelerate you faster even though you may feel the 12T is accelerating faster. A 12T is almost impossible to beat for woods riding though, I feel that its spot on. These are my experiences and I know they go against the general consensus but the experiments don't lie. Top speed for the 12T will still be over 70mph with your setup.