I find that being on a team for a challenge a good thing. There's accountability, as well as goals to be met. Other people are counting on me to do well, & I don't want to let them down. I know that each other member is working hard as well, & there is the built in motivation to help the team win.
There's a built-in support system in a challenge team, & people are willing to work together to win & to do as best as possible. Encouragement is rampant, & that's a good way to keep motivated. Also, as I already said, I don't want to let my team members down, & that's one of the best motivators.