Sorry if you're already doing any of the below suggestions haha, but I thought I'd help contribute too. :) Sorry for the incoming essay lol.

It helps having an end-game plan in mind when GMing I'd think, i.e. somewhat of a clear direction with where you wanna go in the end and a general path you wanna go in. (I've seen a lot of people just start up RPs because they've got a really cool idea to go with - then they lose direction after getting passed the introductory stage.) At the same time, having enough leeway that everyone can experiment and pitch in in the middle helps to keep you the GM interested as well. I've not solo-GMed before because I'm between too picky and too nice, so it would not end well at all lol, but before my friends introduced me to group RPing I always did 1x1 and contributed to the world/setup/plot etc. equally with another person. (What Wizard said too about leaving an idea then going back to it later is good too!)

As mentioned earlier, communication is key too (HeySuess and Dervish make really good points there). So while you have a direction and some basics of a world/setting setup, pushing or insisting for others to contribute their own things keeps things interesting as well. I can't stand RPing with people who do nothing but just react to what the GM's done rather than help push forward with the plot/setting/interactions/whatever. The GM in another RP I'm in also spends some chapters co-GMing with one of his players at a time, so it not only gives the GM a bit of a break regarding always having to come up with new ideas, but it keeps the players themselves chipper and feeling like they have a legitimate role in the RP as well.

The OOC doesn't even have to be about the RP itself; it gets exhausting if it's the only thing on your mind. Just have a group of friendly people to actually have some conversations with between posts. Some nonsense about what music everyone's listening to or what game everyone's been playing lately... starting some tv-show thing where everyone watches like two eps of a series a week. Stuff like that :)

Sometimes the blame for the boredom also falls on your partner/s too, not because you're actually bored with your own idea though it seems like it. Which is again why I said I'd be terribad at GMing because I don't have the guts to tell someone to either act up or buzz off when I'm frustrated with like other GMs I know can. XD; If you ever start another RP you're gonna GM, you could also snoop around your players' activity level (if they're not new) to see if they're chatty on other OOCs or whatever. So you have an idea of what to expect from them too? idk. That sounds a bit stalkerish lol, but it's saved my hide a few times.

Hope something here helps :)

(Also sorry, was snoopin around the BL3 thread during my break and thought I'd comment on this thread too. :P)