I've never tried this on my bike saddle, though I wish I had some the other day when apparently some of my BikeLust made it onto my saddle--took 30 miles or so before it felt rubbed off. But I swear by this for my equestrian saddle. It probably grips leather to leather best, but it says it works on fabrics. I'd apply it to the saddle--it'll go on better than on your shorts.

http://www.sadl-tite.co.uk/