Over time, and a lot of use, most F-clamps start to slip while tightening. If you use cheap clamps, as I did, you can have the same problem.
In this instructable you can read/see what you can do to fix the slipping.
Step 1: What You Need
All you need is a triangular file.
If you have a bench vise it is the most easy way to work. If you don't have one, you can tighten the F-clamp to a solid piece of wood or metal, so that the clamp is horizontal.
Step 2: Start Filing
Put the F-clamp in the vise and start filing. Just deepen the grooves in the clamp. I did 3 strokes in each groove.
As soon as you've finished the outer side of the clamp, just turn over the clamp and do the inside as well. You will notice your clamp has more grip after this filing.
All needed to get some more life out of your F-clamp was 5 minutes of filing, no extra cost.