Sick of the commercials
Seatbelts are you friend, wear them.
But, there should be no law requiring you to do so. You are in no way infringing upon anyone else's rights by not buckling up, the government has no business requiring you to.
I think the cost control argument is a dangerous one. As others have mentioned, plenty of other behaviors are likely to increase your need for expensive medical care, we don't need to government passing laws aimed at keeping insurance premiums down, that's not what they do (or should do, they kinda think they do). What about skiing? That's apt to land you in a hospital. High school athletes get hurt all the time, should we outlaw those? People that don't exercise cost us plenty with all their problems later in life. And the most expensive population in terms of medical costs? Old people. Better pass a law against that.
One solution is to do what we do with smokers, tax tobacco products out the yingyang to 1) provide additional incentive to quit and 2) help recoup some of the costs of treating the morons' lung cancer. But really, I don't think we want to government playing any part at all. If it's an insurance issue, let insurance deal with it. How about a policy that says if you don't wear a seatbelt and are injured your insurance company can deny your claim? That's a little harsh, but maybe they cover far less. Smokers pay a higher premium for the very reasons we've been discussing.
A famous man I can't remember right now said there can be no freedom without the freedom to be stupid. So let people be stupid, but let there be consequences to their stupidity. Then people are free to choose. Not society's job to protect people from themselves.
<edit>Oh, and this one time, I was driving through Kentucky and got pulled over for speeding. Well, I knew the guy was going to ask for license and registration, right? So I fished my wallet out of my back pocket and found the registration and insurance card in the glove compartment. So when the guy gets up to my car I'm ready, but had of course taken my seatbelt off so I could get my wallet out of my pocket. A$$#%^& wrote me a ticket for not having it on.