Do Not Procrastinate
Of course you've heard this a million times before, but avoiding procrastination is the best thing you can do for yourself in terms of avoiding stress. Ironically, stressed out students usually turn to various forms of procrastination to alleviate stress. Taking breaks is okay, even necessary, but wandering away from a project for hours to zone out in front of TV or to distract your friends down the hall from their projects is only going to lead to more stress when you finally have to confront the realities of your homework situation.
To avoid stress, stay on top of your homework. Do not let deadlines sneak up on you. If your imminent deadline is what's stressing you out, the only way for you to deal with the stress is to confront it. Once you start chipping away at whatever it is you do not want to do, you're sure to find that it's not as bad as you thought it would be.
Take Breaks That Refresh You
After you've gotten some work done, it's important to reward yourself with time away from what you're working on. If you plan your breaks, you will find your work less stressful to begin with. And because you know there's an end in sight, you will probably work more efficiently, too.
When you've earned your break, do something that's going to make you feel good. Going for a walk is a great option because it will clear your head and give you some exercise, as well as some perspective on your work. In fact, many students find that if they are having trouble with an assignment, that the solution to their problems suggest itself if they go for a walk. Once you've started to work on something, even when you walk away, your unconscious mind keeps going, and sometimes rewards your efforts with the answer you've been struggling over when you least expect it.
Exercise in general is great for a break, and for a way to avoid stress. The exercise you do works out tension you've built up your muscles, and the endorphins your body releases after a great workout will naturally make you feel great.
If you do not feel like exercising, do something else that you do enjoy. Call your mother or your best friend from high school, catch up on emails, or read a book for pleasure. Just remember that you're on a break, not a vacation. Set limits for yourself. Maybe talk for one half hour, and then go back to work with the promise of another break in a couple of hours.