I found the office type black clamps too small for my countertop. Now I use these:
http://www.menards.com/main/tools-ha...914-c-9135.htm
They are *very* easy to use. There are smaller versions, but I like these because they will handle any thickness of table I have come across. They are also gentle because of the ratcheting mechanism and plastic ends, so will not damage a table. Found them in our local Menard's store for under $5 each. Four of them is enough for me to do a lap size quilt. For larger quilts, six seems to be enough.