Not sure if this is exactly what you mean, but I avoid burn out by raiding with my main a few days a week, 5-boxing a bit and make sure to take at least 2 days off from WoW in general to do other stuff. I will get burned out if I do too much of the same thing.

As far as being bored while leveling up, I was watching TV or movies at the same time, so I didn't think much about the mind numbing collection quests.