I'm using the mesh drywall sandpaper. I'm 225 with 10" and they haven't moved. You only need a 1"x1" square on bottom of each clamp and tighten to spec. Tighten top bolts (towards front wheel) first.

Drilling just seems to permanent. What if you want to change bar position? You going to keep drilling?

 

I cut some strips of a beer can to put under mine and it fixed me up. Inner tube sounds even better though.

 

Every couple of months I would hit a major expansion joint or something just as trivial and back would come my stock 10'' mini-apes.

My eventual solution?

I bought a longer Allen wrench!
That's it!

I was able to torque the bolts down just that much tighter and I haven't had issues since.
It was a good investment that fixed an annoying on-going problem.
If that didn't work I was going to use rubber sheeting I brought home from work a few years back but I didn't really want to go there if I didn't have to.

 

so my buddy doesn't want to drill my bars. he's afraid of a stress fracture...so i'll try some of the ideas posted here first...thanks everyone