Keep core temps low (subjective, but generally below 80C for 24/7 operation. Although some have stories of running cards at core temps near 90-100C, none of them would truly recommend that practice in general). Do note that you generally have diminishing returns on this (for example, cooling from 60C to 55C probably offers smaller lifetime benefits than 80C down to 75C).
Avoid unnecessary strain on the fans (no need to run fans at 100% just to get below 60C, or whatever). Use cheap box fans to supplement, but avoid causing turbulence with the integrated fans. Also remove dust buildups as-needed (varies based on the air quality of your mining location)
Avoid excessive overclocking. For example, excessive memory overclocks can cause artifacting, which is a symptom of an unstable overclock that may cause permanent damage, and this symptom might not be noticed without having a display connected to each graphics card.
Use a quality PSU. A PSU made by a reputable OEM (particularly Seasonic, FSP, SuperFlower, and some others) generally has a better chance of consistently providing clean power to your cards, rather than flaky no-name PSUs which may have more irregularities.
I have 7700 and 7800 series cards from 2012 that still work fine today (used for mining from mid-2012 to early-2015, then fired up again in 2017.). A good friend of mine still has some 5000 series cards from 2010 that are still operational (but not actually used for mining anymore).
Thanks for the info; Very usefull. They run currently at 75 degrees and I have additional fans installed. So I guess it should be fine like that).