If you are a person who is brave enough to test the waters of stretching your own canvas, a pair of these are essential. They allow you to evenly grip the fabric, pull it firmly all with your weaker hand, leaving your dominant hand free to staple the fabric to the stretcher bars.
These can range in price from approximately $15.00 to about $60.00 - go with a nice heavy construction and make sure the surface doesn't have rough, unfinished patches. If you have a decent art store nearby, go in and test out a few to see that they fit nicely in your hand and you can operate them easily. They are worth every penny.
Even if you are not the type to stretch your own canvas, a pair of these sitting on your desk can discourage people from making last minute demands of your creative ability. Simply pick them up and slowly open and close them, be sure to frown slightly and make direct contact. The clamping sound will discourage even the most insistant person. Trust me.