Rule of thumb, I have been mining for almost 4 years with 120 plus cards. Lot`s of expensive lessons learnt!

1. Power supply, never push your power supply to it`s limit. Sweet spot is around 60% utilization.
2. Dust is a card killer, make sure you maintain dust as well as you can
3. Temperatures, place your rigs in a well ventilated room. Try remove the heat generated by the cards (Extractor fan).
4. Don`t be an IDIOT and OC your cards to the max. Loads of people are chasing hashrate instead of finding a sweet spot.. pushing a card to just under the point where it crashes is really idiotic. Your card might cope for a couple of months but it will die on you after a while when pushed it to the max.

Cheers