Dildo Harnesses
The two main types of dildo harnesses are one-strap and two-strap. One-strap harnesses slip on like a G-string: they give you a more intimate feeling with the dildo but are less adjustable. Two-strap harnesses strap under your butt (where it meets the thigh) and leave your private parts exposed.
If you'd like to add more power to your thrusts, a thigh harness might be just the ticket. Unlike standard harnesses, a thigh harness straps onto your thigh and leaves your crotch free and unencumbered.
Whatever the harness, the most important feature in a harness is the O ring where the dildo is mounted. Most harnesses use a 1 ¾" inch ring, which means a dildo with a higher diameter than that won't fit. If your chosen toy doesn't fit, choose a harness with replaceable rings.
In any case, material is less important in a harness than in the dildo, it's what's inside that counts. If you want waterproof, go for rubber or nylon. Otherwise, it's a matter of style (something leather and vinyl harnesses have in spades).