Twilight, has luna or celestia told you how they get their manes to flow like they do?
Well, that’s just an effect of their control over their respective heavenly bodies. Performing their duties rejuvenates them, and infuses their bodies with magic which makes them become tied to their purposes even in how they appear. It is a phenomenon known as forma officium, and is related to the magic of cutie marks (which makes images related to one’s talent appear on the flank).
Of course, it is also a choice, they could elect to not have such a mane. And there’s bound to be some more mundane mane maintenance involved, though it’s not like I spy on them in the bathroom so I don’t know what’s involved with that.
Seems like such a complex mane would be a lot of hassle to keep organized. I like keeping mine simple, just have to brush it in the morning and wash it regularly, and it’s neat and tidy!