on your press, take two good size round bar (1.5 to 2") and them about 6 inches apart on the press, then very carefully lay the leaf spring across the two bars, and have the press push down on the leaf, do this all across the length of the spring, moving it about a inch from each press down, you might not see any difference on the first pass.
there was some pictures of how to do it on here or somewhere, I just remember seeing it on someone else post, they where re-arching a leaf for a old hot rod using a homemade press, I gave it a try on a friends shop press, and I got 2" lift from doing this.
I'll try to find some pictures and post them, just be sure to lay your leafs on their side and using a marker or something, and draw a line on the floor to show how much bend you have now and after on the springs, I used a marker and my garage floor, that way I could walk across it with out messing up my outline.
Robert