This may sound odd, but taking regular breaks really helps me. I get focused on a task and it get where I am just grinding trying to focus and will myself to get it done.
Not only allow yourself regular breaks, but make yourself take them. Get up and walk around even to get water and visit the bathroom.
If I am going into exam week or something really stressful, I try and plan a reward time out. 2 hours at the movie theater or an hour eating lunch. No school. No work. Try not to stress during these times by going over what you have left to do. Just be there and breathe and destress. I have found my studying goes much better with realistic expectations and timed breaks. I have even set my iPhone to ring or sticky notes of times to break. Work smarter, not harder.