Bring a pair of longish cotton socks with you to your elliptical, treadmill or exercise bike of choice, and put the socks over whatever hand-bar you're going to grasp onto. If it's an elliptical you're using, you can slip the socks down over the arm grips; for the treadmill, you can put them over your own hands or tie them in grippable knots around the horizontal bar. Holding onto a cotton-covered bar is SO much more tractioned, comfortable and DRY than a slippy, sweaty piece of plastic!!
I do this on my home elliptical all the time! I used tie-dyed socks -- adds color to my workout! :)