Everyone's creativity runs on different cycles, so everyone's answer might be totally different. For me, beyond the obvious (playing with fresh/new gear, plug-ins, or soundmaking objects), there are three tricks that usually help.
The first is to get in tune with my own creative rhythms. I've noticed that my creativity increases under certain conditions and times of day, so I'll rejigger my schedule to best accomodate when I know I'm at my most mentally fluid; it certainly isn't the middle of the afternoon, I'll tell you that much. I'll save my afternoons for repetitive tasks, and let myself really rip, say, first thing in the morning, when I'm still groggy, can free-associate easily, and can think creatively with fewer preconceptions or distractions.
The second is to get away from the problem for a while. I learned in art school to work on multiple pieces at once, and literally to turn the canvas of one piece against the wall, out of sight, while working on another. In this way, I purposefully interleave tasks, errands, and even projects so that I don't grind down on one thing for hours and hours. Taking a bike ride in the middle of the day, running an errand, or even taking a shower can give some distance from the challenge at hand. Distance = perspective = objectivity. For me, exercise almost always helps.
Finally, I free associate in order to re-frame the problem. Sometimes this is mind-mapping, sometimes it's just lists of words. But for me, working BACKWARDS from the desired emotional outcome and working towards the solution is the best way to go.
There are a variety of interesting unblocking "accessories" out there that some use, from Eno's "Oblique Strategies" to Thinkertoys. I've not used them myself, but they serve the same function of giving you some perspective and helping to re-frame the problem to facilitate fresh thinking.