I've been having problems keeping the Dunlops from slipping off. I tried Earl's suggestion for the plastic thumb pick of scoring the inside and it works well. So I tried various ways to get the same effect in the metal ones. Right now I am experimenting with a small bit of adhesive tape inside them, and so far it's working a bit better. I'll have to try it longer to see if it is a longer term solution.

Any other suggestions?