I think the trick to overcoming burnout is making a change. Whether it's to your route, the bike you ride, your pace... whatever, change it up. There are times when the thought of getting on my fixie and going b***s out for an adrenaline rush on my morning commute just doesn't appeal to me so I pull out the heavy MTB, take a different route to work at a relaxed pace and by the following day I can't wait to get back on the fixie.