Hmmmm, I'm with jimc101. I think the OP might be talking about housing ferules with the mention of plastic ones.

Factory ones often come crimped but I've never tried to crimp one at home. Luckily I only have to change cables occasionally so the worry about dropping ferules isn't that great. I guess you could put a drop of super glue on them...

which also works nicely in place of cable end caps.