Measure. Measure how much you actually get done in each sprint. If the team is regularly saying that they can do 20 points of work, but they only get 15 done,

Measure. Measure how much you actually get done in each sprint. If the team is regularly saying that they can do 20 points of work, but they only get 15 done, then they should only be signing up for 15 points of work in each sprint, no matter how much they think they can do 20.