I'm sure you have your own story about how you or an acquaintance discovered the usefulness of regular collaboration in learning physics. Our challenge, as physics teachers, is to push our students toward their own epiphany sooner rather than later.
In recent years I've taken the bull by the horns, and simply required nightly collaboration. Students must write down the name of the student(s) with whom they discussed the problems, or at least checked their answers. No collaboration = not full credit. Such a requirement is easily workable in a boarding school, where a classmate is guaranteed to live no more than 15 yards away. It's workable in a day school, too, in the age of email and social media. Discussion via facebook or twitter is okay by me.