You can go to the dollar store get the table cloth clips that hold the table cloth down on a picnic table. They are plastic, but they work for a long time.

I use clamps from my workshop to hold down my long rulers on my cutting table to cut batting and trim completed quilts. They're very sturdy as they're metal and have rubber covers on the portion that touches the fabric. It takes both my hands at times to open them to clamp over so that tells you just how strong they are. They're called spring clamps and this page on Harbor Freight shows the 2 types I have.

http://www.harborfreight.com/catalog.../?p=2&q=clamps I use the ones that are all metal in my sewing room.

My husband has these and they work great for just about anything. They should hold the quilt to the table very nicely.
They adjust very easily.

Very good if you are using big clips, these are much easier on you hands. If you have to go buy some, test them in the store before purchasing. Some can be quite stiff.

These clamps are too big and bulky (cumbersome). You will be fighting them. Mandyrose posted a photo of the clamps that are easy and hold the fabric very well. Again, you must have a thin table top for these, but they are preferable to me. I would rather invest in the correct table (Office Depot, etc) than try to fight these large clamps. I have tried some similar clamps and they don't hold the fabric in place very well because they don't clamp on the table correctly. These clamps are "men clamps"...hahahaha