I can't think that I've ever ruined a Presta valve, although I do remember one that had that screw thing broken off years ago (I don't know who did it). Keep a cap on your valves when not in use.

If using a portable pump, always hold the pump head.

The Lezyne Style mini pumps have a reversible presta/schrader hose which seems like a nice idea. I gave one to my nephew so he can have something good to start with.