Summary

Use a cheap wooden clothespin to keep tension on your belts!

I didn't invent this idea, but I found that using a standard clothespin required a little experimentation so I thought I'd share the technique I used.

This is how I keep tension on my Wilson printer. Did I mention that it is extremely cheap?

Instructions

Acquire clothespin. Mine came in a pack of 50 for about $1.
Disassemble the clothespin.
Using needle nose pliers, straighten the ends of the spring.
Gently spread apart the spring so that there is about 90 degrees offset between the pointy ends.
Reshape the spring so that there is a double-bend on each end, forming a U shape that will prevent the belt from slipping off.
To install the tensioner, first move the axis being tensioned to one end (for example, push the print bed all the way to the back for the Y axis). Identify which side of the belt will move away from the opposite end as the bed moves, then place the tensioner on the belt as shown. Proper location of the tensioner on the belt is important so that the tensioner doesn't get shoved into the idler or motor ends.

The belt should be captured within the U-bend that you put in the spring and under tension.

Simple clothespin belt tensioner