I listened to a number of podcasts on keyword selection in Amazon listing, and I can condense them into a single phrase:

Keyword Stuffing Hurts Your Listings

It used to be that you could cram 1000 characters in your listings back end, but that number has been reduced to 250.  Google severely penalizes sites that keyword stuff, and Amazon is doing the same with listings.

If you control a listing, pick wisely, make few changes, observe the results, repeat.