Personally, I've done it a couple different ways, albeit slower ways than say, questing/grinding 24-7. (The latter I do NOT recommend!)
The first: I went to the previous xpac lands and farmed ore/herbs. Note that though they are 'grey', I.E. you do not raise your herb/mining levels when you farm them, they still yield XP!
A couple (heavily truncated) notes on the first method: Less people in old content. You can't fly until level 90 in MoP - flying in the previous (Cata) content beats ground navigation in new content for many (some should be quite obvious..) reasons. Old mats can also sell better than new mats.
The second: As previously mentioned, Archaeology! You're flying around in the wide world...of Warcraft, where the only people you'll see are those also digging the sites. Let's face it - some would rather pour acid on their nether regions than level their Arch. But this secondary profession not only yields a decent amount of XP at a moderate pace, but the solved items are pretty rad. Well, at least most of them. And all the gray items? Well they can sell for 1 to 100gold (a piece!)..after a few hours you'll easily have over 1K.
It's pretty neat to log back on in the morning and seeing a few legendary BOA items, a few new pets, and some novelty items in your bags.
I recommend getting the LogMeOut plugin for *any* use of HB. Set it to run for a few hours, and it'll automagically log you out. Bot in moderation!
EDIT: At the moment, Arch is yielding around 2M/hour.