I put SW Motech 30mm risers on mine. Top clamp works fine, however, the two sets of bolts they give you are both too long. I now have 5 washers on each one and the handlebars still rotate a bit under stress. I'm going to have to find a source for stainless steel hex-head bolts in the correct size.
See the pictures/description here:
Handlebar risers on
If all you want is more pullback, you can get a new set of handlebars for half the cost of the Motech risers, $20 vs. $40 for the risers.