Well, for school, if I work on one subject too long, I need a break. I play piano or switch to another subject.
As far as projects go, I actually bounce between a number of them. YouTube videos? I have about five I'm working on off and on. When I feel interested in one I do it. But writing? I, uh... may or may not have over fifteen different projects I bounce between.
Sometimes you just have to force it. Like, say, cramming in two weeks worth of science into about 4 hours from 8PM to 12AM. (This happened to me last night. Would not recommend.)
But the motivation actually just comes and goes for me. (Which would explain my +12 writing projects) However, I'm learning that when I have the inspiration or motivation to work on something, do it. Get it done ASAP. As a writer, I try to wake up 30 minutes before I normally would and set that time aside to write, because I know that if I don't get the project done quickly, I'll loose motivation on it and it'll go unfinished for a while, if not forever.
Hope this helps! (I also really hope I didn't misinterpret the question, lol.)