Regarding motivation for exercise, I find competition & praise is particularly helpful, perhaps especially for those of us of the male persuasion.

Fitocracy is kind of fun and has various challenges. Last summer I did various cycling & running challenges and eventually ended up so motivated that I completed a 100 km ride and a marathon, essentially for fun.

Alternately you could make bets with a friend about some well-defined goal like being able to do 100 pushups at a sitting.