http://static.grainger.com/rp/s/is/image/Grainger/4DV33_AS01?$zmmain$ $10 or so from Grainger. There is not a lot of hammering area, but they have always worked for me. Heck, I beat on the nuts as much as the clamps base. Doesn't seem to hurt them none. The pair I have are probably 40 years old and have pulled a good dozen sets of bars.
As mentioned above, I loosen the lower control arm nut and pry the the lower control arm back and whack the clamps.
Although it is probably not necessary, I clamp 2 clamps back to back against each other for more clamping force. One would likely be enough for most bars.
