Jigs and fixtures are a whole art in themselves. So much depends on the work you want to do, and the part in question. The speed necessary to complete the work is a factor, as is the time needed to make the jig versus the amount of parts to be made.
I work in sheet metal a lot professionally, and holding it is frequently a challenge. A LOT depends on the thickness of the part. When I can I'll use soft jaws, but I've made specialized clamps to hold areas of the part. I also like using 90 degree angle aluminum stock with the "mouth" of the V pointing down towards the part. I have a bunch of big rollers at work so I'll frequently put an intentional bow in it so that the ends won't touch until clamp pressure is put on it. This gets more even pressure across the part.
Another thing I do a lot is to mill out a very shallow depression in a piece of sacrificial stock, and drill and tap holes around the periphery to hold small strap type clamps. These usually stay well out of the way of the tool and holder.
Many use vac chucks with thin parts like these, it's something I keep meaning to put together at work but never seem to get around to it.