I would say the key to increase productivity while studying is to avoid distractions as much as possible. Limit your screen times!! Do not pull long hours - work out a schedule for every day and stick to it, ensure you take enough 5-10 minutes breaks.
When I was in college, I used to list down the topics / areas that I need to cover in a particular day, further I used to break those topics down in small segments which I covered in sets of 30 minutes followed by 10 minutes break. Then the next topic.
The key here is also to start working on your study plans a couple weeks before your exams - that way you feel less pressure and are able to give enough time to each area.
Once you are done for the day, make it a point that you do something you like - could be a sport, spending time with someone, working out, anything you like!
This is something which is very generic. Once you try doing this you yourself will be able to come up with a method that you feel is good for you.
Hope this helps :)