Hit the reset button. Usually I get stuck because my mind becomes hyper-focused on something that isn't really that important. I take a break of some kind, that seems about appropriate to the problem: feeling frustrated at work? 15 minutes away from the desk. Bored by everything to do in my house? Go out for a run.
If that doesn't work, I then break the task down into lists of its components until I have reached a point where one component seems easily doable - in fact, often I like to do with "laughably feasible." Once you get started, it's a lot easier to keep going.
When writing, sometimes I just vomit words onto the page, especially if it is a first draft. I usually find I get where I need to start from once I've thrown up a page or so. It's okay if it's trash. You have to figure out where you want to start from first, and sometimes you can reach that mentally, but sometimes you need to get in there and write until the beginning just finds its way out.
It sounds like you are writing prose pieces. Have you tried approaching them by building skeletons: write out the first sentence, follow with your thesis or major point (eventually, these two sentences will be filled in with your first full paragraph). Then write your first topic sentence. (This will be your second paragraph.) Write your conclusion for that topic. Progress. I did this for the GREs and hey, at least it fooled them - I guess I can brag I got a perfect score.
Remember, you might not be happy with your writing, but that doesn't mean it's worthless or can't satisfy everyone else. It is more important to start getting the point down than say it perfectly.
If you are having trouble being creative, what I like to do is find my craziest idea, and then stack on it so things become progressively crazier. Somehow, it works, at least for poems. Just go all the way out there, balls to the wall. It works more often than it don't.