I've used neoprene saddle pads and girths before and I think the material is missing its calling by being marketed as anti slipping. They would make so much more money if they made it into cruppers and said something more like "lubricates with sweat to prevent rubbing and allow freedom of movement." Hahaha! The only piece of tack I have that has neoprene padding on it is the breast plate for exactly this reason. When it gets wet, it seems to float on a thin film of water. Very useful for some applications, but not for gripping. :P
This is what I have right now. A Toklat Woolback dressage girth: Toklat Woolback Dressage Girth
This thing was definitely worth saving up for. My horse's back is very round, and even with a well fitted saddle and a leather girth we were still having slipping issues (which is not at all pleasant when your green horse does the 'drop out from underneath of you and pop back up three yards to the left' spook at the drop of a hat). I did some research and decided that I should try a wool pile girth. This one gave me an edge on the slipping problem because the wool grips onto horse hair (in a subtle way; it's not pulling on it). Plus it has a large surface area and it is incredibly comfortable and pressure distributing. At first I thought it would be hot, but the wool kind of wicks the sweat away.
Can you tell that I'm enamored with it? ;)