Roxio Spin Doctor is an app that is often overlooked for this task (especially in OSX). First, expand the mp3 file to AIFF, open it in Spin Doctor, and you can insert track breaks into it. It even has a feature to auto insert breaks, but you have to do some tinkering by hand normally anyway. You then save the AIFF and bring it into Jam or Toast and it will burn with track breaks.
I have tried serveral methods including Audion's mp3 editor, but the seems to be the fastest and easiest way. If you own Toast, chances are Spin Doctor is installed already as well.

