Inspired by Roboteernat's super simple Paperclip belt tensioner (http://www.thingiverse.com/thing:4535), but not quite getting the results I wanted, I came up with this super easy and inexpensive solution to tighten my belts without new parts and, most importantly, with disassembling my printer!
You'll need 3 cable (zip) ties, two of them should be small enough to fit between the teeth of your belt and the third sould be long enough to take up the slack in your belt.
Loosely zip the two small ties around your belt, then zip the larger one though them as loosely as possible. Position the large tie along the flat side of the belt, giving some thought to the position of the ratchet (center in along the top lenght is likely best, the photo's a good example of what not to do). The two smaller ties can then be aligned between two teeth as far apart as possble and tightened enough to grab the belt.
You can now tighten you belt by tightening the large tie!
Since you can position them ties anywhere in the belt and once trimmed they have a very low profile, you can use this methid in tight spots like a Prusa Y-stage with little difficulty.
The only down side is that it's a one way adjustment. If you get it too tight, you may be able to get a needle or small screw drive into the ratchet box to release it, or cut it off and try again. For the price, it's still a good deal.
*(Cable tie model borrowed from http://grabcad.com/library/cable-tie-mount without permission for illustration only, I wouldn't suggest printing it - especially if you belts ar loose!)