I know there are several products out there to do this, but I thought I would share at little tip that I use to keep my parallels in place during the machining process.
I use (4) little Neodymium Cylinder Magnets to hold the parallels in place. I realize they will not work in all your projects but will work well for most of them. They won't work with Aluminum soft jaws or if you are trying to hold very narrow parts. These little powerful magnets are avaiable in different sizes and shapes on E-Bay. They have lots of uses around the shop. I bought some because if visited fret's.com website and looked at his machine shop photo gallery. He has done some very nice stuff with magnets.
Attached are a few pictures for your amusement. Enjoy!
John