I don't drive (and this would change if I get a motorbike) but I pretty much always wear a seatbelt.
Only time I don't are on buses/coaches (buses don't have them, coaches you're pretty much fucked anyway) and in taxis for some reason (London cab style ones - not the more "car-like" ones).
But whenever I'm in a car I always wear the seatbelt. I like my insides inside, and all those "My friend's, friend's so-and-so would have died if wearing a belt" I personally think is a load of bullshit. Those occurrences are so extremely rare that you can't "rely" on them like you can a belt (I remember one accident where a guy survived because he was thrown clear of the car without his belt. Paralysed from the waist down, but alive). But I know that at least 1000 times more people that have said they're alive because of a belt.
Being lazy, thinking it's uncool or uncomfortable are no excuse. Even if you don't value your own life, or are sitting in the back, you can still kill other people, either in your car or the car you hit, because you become a projectile (there was a fantastic advert a fair few years ago in the UK where a girl sitting in the back isn't wearing a seatbelt, and kills her brother in the front).