the only way i have EVER been able to do it, is to put try and put the cap on by hand, once you get it in position use a rag thats been foled a few times over the cap, then tap it on with a hammer!!

I know it sounds quite bad, but as long as you are firm and can hit the cap square it should pop back on OK.

PS if you allways hit your thumb when using a hammer, probably best that you dont try doing the above method!!