I use a suction device on my slave and it works great. It was in a kit that I picked up at the local toy store. It also came with those little rubber bands. Actually little "o" rings is more what they are.

The suction device fits right over the nipple and makes it very erect. At that point you slip the band off the suction tube and onto the nipple. It works great and was only about five bucks.

Barton