The most unhelpful advice is to dig in and work on the things causing you stress (provided that those things are time-constraint-based things, like classwork). The sooner you get things off your plate, the less stress you will feel. It's the good ol' Nike commercial slogan: Just Do It. There's always the addendum piece of advice to that which is to break things down into "more manageable pieces" and start knocking those off one at a time, with each piece knocked off decreasing your stress just a little bit more.
Oh, and the WORST advice is to "stop thinking about it - you're just stressing yourself out."
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Recognize that Stress = Fear of [something]
So, what I like to do is take a moment to figure out why I'm feeling stressed in the first place, which sometimes requires a lot of honesty and digging in my mind where my mind doesn't want to go, which is always towards something that is causing me some form of fear and anxiety. Most times it's because I'm upset with myself for procrastinating something in the first place (which I procrastinate things quite often). The reason I'm upset isn't exactly because I procrastinated as much as now I am potentially facing consequences for that procrastination which I don't want to face, which in the case of when I was in college equated to a late, sleepless night of cramming for a test and running the risk of getting a bad grade.
Stress = Fear of bad grade
Now, that example is me and my tendency to procrastinate. Your cause of stress may be something different, but the fear driving it is what's important.
For me, once I accept that the worst possible result CAN POSSIBLY RESULT, then I usually start to relax and let go of the stress. I guess it falls back to asking, "What's the worst that could happen," and as long as the answer isn't death (and sometimes even if it is), then once I accept it, I feel better.
THEN, once I feel better, I dig in and start getting things knocked out. Not only do I feel less stressed, but I tend to also feel energized to move forward in a very determined way, which almost always equates with the final result not being near as bad as my worst case scenario.