I'm playing intramural softball now. Our first practice was today and our first game is tomorrow. Although we won't be practicing much, I think that there's still enough incentive to be halfway serious about the games out of respect for my teammates. I guess it's hard to play softball without a team...but you can run with or without a team.
And when I run without a team, I feel that it's not as bad if I slack because I'm not letting my teammates down. Having somebody else there for your practices/workouts holds you more accountable. This effect isn't constrained to the realm of sports, either. I have never done theater, but I think that this applies. If you stretch your imagination, it could work with grades in school (who is your team? the people in your study group.).
Even though I think I'm a fairly self-motivated person, having others to let down is an incentive to do it right. So, if you're slacking, try getting someone else (who you care about) to hold you accountable.