I wanted to know if there's anything available to keep the rubber boots covering the CV joints from drying out... Obviously there's grease inside, but what about keeping the exterior from hardening/drying out and cracking due to flexing and stretching from normal wear and tear.

 

I don't think it's really necessary but if you want to be anal about it (and I mean that only in a positive sense), belt dressing would probably do the trick. It keeps rubber belts nice and pliable so it should do the trick there too.