Step 1) Go on Reddit for 3 hours
Step 2) Realize you just spent a ton of time doing nothing
Step 3) Get more stressed
Step 4) Do two problems to get started up again
Step 5) Take a 10 minute Reddit break because you are proud of yourself.
Step 6) Repeat Steps 1-5.
Honestly though, I just reassure myself that my plan will prepare me. Any of the above choices work though... Exercise, naps, snack break, video games... just make sure you set a time limit and are disciplined enough to stick to it (I'm pretty disciplined myself, but I'll occasionally take a 30 minute break instead of a 10 minute break and I feel awful after... Stick to your schedule!).