Rather than paying out the nose for them($6 for ten), you can make tensioning springs for timing belts using clothespins. At Wally-World a huge pack of them was $0.97,http://www.walmart.com/ip/Mainstays-Clothespins-50-Pack/21081344, but many already have some laying around(I did but they were rusted from use outside).
Be careful cause these springs are deadly, we maybe not deadly but they will hurt like the Dickens if you pinch yourself.
Step 1: Separate Spring From Wood
Easiest way to remove the wood is to press along the side of one half and slide the wooden slivers while gripping the spring. Alternative would be to pile a bunch of them on a bar-b-que pit and make a mini funeral pyre but that's a little bit overkill.
Step 2: Bending
After removing the wooden bits, the two "arms" of the torsion spring will naturally come together. To make tensioners, we need to bend the arms back to a more usable angle. For my k800 printer(kossel mini) I'm using the GT2 belt and pulleys, and the spring arms at roughly 120deg angle. The exact angle doesn't matter, the good ole guess-&-check method works best.
Step 3: Install
To install the tensioners place one arm under(or over) the belt and push the spring with one hand while using the second to fit the belt over the other arm. The force of the torsion spring trying to bring its arms back together creates an arc in the timing belt which takes up slack. This way your belts will be tight enough that they don't skip over teeth on your pulleys, and will greatly reduce backlash.