Adjustable C clamps are some of the most useful tools in your workshop. Here are a few tips to get even more mileage out of them:
To hold a piece of material in a vertical position, take one clamp and attach it to the material and then use a second clamp to attach the first clamp to the workbench.
If you don't have a clamp that's long enough for the job, you can combine two short clamps into one longer one.
If you want to clamp something like a veneer patch in the center of a large panel, but your clamps just aren't deep enough, place a wooden block on the pressure point, lay a longer strip on top of that, and then simply attach a clamp to each end.