For me it just depends what I'm grinding - some things that I want I end up thinking are worth the grind.
A good example is when I did Loremaster on my shammy, I was so bored many times but I either kept going because it was something I wanted to work on and I thought the achievement (not just the proper ingame achievement, but my own sense of it) was worth the work, or at times I just took a short break to watch TV or play TF2 or something.
Another example is less apparent in WotLK but something I did quite a bit in tBC, where I'd farm/grind mats/rep/similar stuff for things like mounts or gear, but again I did it because the end result was always something I saw to be worth the work.
Put simply, I tend to see the game less as a boring grind and instead as something that you need to put the work into in order to get the best from it.
That isn't to say that I don't sometimes get bored, but like I said, when those times come I just log off and play something else or watch TV or something.