I lost my 3rd wagon wheel center cap last night. I m sick of paying $20 or so bucks on ebay for a used cap. I am have a new set of the plastic rings that I'm going to put on. Don't know if that will help.  

 

Anyone have a trick to keep them from coming off?

Install with the wheel off. Push the little tabs out until they grab the wheel opening firmly. Then tap slightly on the back side pushing the ends of the tabs further out to grab the wheel center opening. I had lost all my plastic pieces and using this method lost zero center caps.