Hmmm... two words man:
Peer Pressure
It works wonders. If the majority of the people are pulling their weight, and see some leech goofing off, make it a commonly accepted practice to ostracize them. The problem will promptly solve itself. No one wants The Man telling them what to do! If their friends make it clear that what they are doing in unacceptable, they'll change in a hurry.
Just mold your team's social dynamics, and remember that kids ridiculing one another can be a good thing at times. Make slacking off taboo!
That said, I must say we are a very small team where most want to work hard. We are fairly relaxed and tolerate some good natured time wasting, but everyone is quick to jump one another if it gets out of hand.
The real danger I could forsee would be a small faction of people united in their slacking. You've really got a problem if this happens. I've never seen this on robotics, but its happened on stagecrew occasionally (and I was usually part of the problem
)