By taking long, long breaks, and by fandom peeking.
I do get burned out. I am burned out right now, as it happens. After nearly four years, I am SO burned out!! But I stick to my guns, because I promised I would finish. And I remind myself of what I love about this gigantic, consuming, wonderful thing.
And yeah, fandom peeking is fun. I read a fair amount of fic outside of Tolkien fandom. And often it is for fandoms that I don’t know anybody active in, or have never participated in. Sometimes I need a total change of pace, a new vista, and a fresh gust of air to blow the Mirkwood cobwebs off my brain.