I don't know how to avoid burning out, but I always know I'm on the verge of burning out if I find myself doing legacy content. Whether it's farming xmog/mounts/achievements, I typically burnout shortly after pursuing that content. Right now I'm barely playing, just waiting for 8.3 to launch. In the meantime I leveled some horde alts to 120 since I already have one of every class at max level for alliance. Outside of that I'll logon to do gold emissaries across all my alts. There honestly isn't much to do unless you have a super active guild and a clique to do content with. Korgath is dying and every guild is transferring off. I just hope blizzard holds true to more realm merges like they stated. Hopefully they happen prior to 8.3 or shadowlands at the very least.