Re: avoiding burnout, I have found that keeping a good variety in my work helps. I freelance, so I don't work at the same theatre all the time. I do theatre, opera, and a little bit of dance- when I am a little burned out on one thing, I focus on one of the others. I've also dabbled in some non-SM jobs that use similar skills- working on festivals, workshops, production managing, etc.
Most importantly, I try to make time for myself. When I was younger, I took dance classes and ASL classes. Now I garden and read and knit. I try not to overbook myself, so that I actually have time to do some of these things. If I have a few weeks off between shows, I don't necessarily try and book those weeks anymore- a mini vacation does wonders for me.
EDIT: Also, make time for friends and family! Doing nothing but work is a surefire way to burn out.
Regarding boredom and dissatisfaction in your work- often I think this happens when you're not challenged in what you're doing. You can look for positions that WILL challenge you- a higher level position at your theatre, working at a bigger theatre, working at a theatre that does different work (i.e. musicals or new plays if you normally do straight plays), working in a different medium (dance, opera, production management). Something that will make you grow (or give you the opportunity to grow). That might be a non-stage management position.