Tommy brought up a good point. I often times also study with classmates and make it a somewhat of a social event. Make sure you study in an environment where a lot of other people study as well.....if I see other people study and I'm just sitting there goofing off, I will feel guilty and study some more. Also, it helps to ask your friends when you come across confusing concepts (altho in dentistry, there's not as much concepts involved and mostly just straight memorization).
Another reason I am motivated in because I've done plenty of procrastination, partying, clubbing, raving, drinking, slacking, and basically anything that has nothing to do with books in my college years. The rebellious little kid inside me finally decided to grow up. I feel that I dont need to repeat all that again, now that I'm in dental school....... although I still go out a lot.....hehe. Also, something that keeps me going is that I want to eventually pursue in a post-graduate speciality program. I think this is ultimate motivating factor cuz they really wont even consider you if your grades are mediocre.
Bottom line is that as long as you are methodical in your approach in studying, you should be fine. In my first 2 years of dental school, I allocated 3 - 4 hours a day on average studying......except friday and saturday nights. Throw in some relaxing time in between studying days to keep yourself sane.