For what it's worth, here's what worked for me:
Plan mini-rewards for yourself. For example, plan on a full week of studying and then one day on the weekend (or even just an afternoon and evening if that's all you can spare), get out and do something fun - go for a hike, to the beach, etc. etc. (all the usual things.) the key is to plan it ahead of time, so you have both the anticipation of FUN THAT'S YET TO COME!! and the knowledge that you have earned your reward and can therefore enjoy yourself without guilt.
Also, I'm not a fan of marathon study sessions. break it up into manageable chunks, and focus on one subject per chunk. (eg 2 hours of o-chem, 30 min break, 2 hrs of physics.)