I get good results from Gorilla glue.

A good glue for plastic will depend on the type of plastic, really. If you've got a acrylic plastic, something like superglue, or model glue would be best, as it actually softens the plastic and melts the edges together. If you've got something like polyetheline or nylon plastic, you'll want an epoxy that will stick to the plastic and then solidify.
Try putting a little drop of super/model glue on the plastic (not onthe edge) and wait for it to dry. If the glue sits on the plastic and hardens, and you can chip it off, then use an epoxy. if it "soaks in" and becomes part of the plastic, then use it to repair the break, and use it sparingly.
But it's tough to beat Gorilla Glue.
