Basically, you're not going to be able to force people to stick together after a game jam, but there's a few ways you can help. Some of these suggestions are for local jams, and some are for online only jams (and some are both)
First, get the participants to exchange contact information (names, emails, phone numbers, social security numbers (not actually social security numbers)). This will at least allow people to look up their teammates in the future if they want to collaborate again. This is also pretty easy to do.
Second, if it's a local Jam, organize game developer meetings in the future, so people can keep those connections fresh, and hopefully gain even more of them.
Finally, if it's an online Jam, establishing a year-round presence through a website, forum, and/or chat room will help keep those connections alive. For instance, Ludum Dare has an IRC chat room that is active all through the year, though it becomes filled with tons of new people during each event.