By the book, the answer to your question should be simple. In order to get sims to commute between cities, you must provide adequate road, rail, sea, or air transport to move them effectively between two cities. You must also provide a commodity or opportunity that is in short supply in their home city, such as employment, education, or shopping.
Simply using the highway to allow sims to travel by car will not produce as good a results as using a Regional Bus Station in each city. For better results, a rail connection will move more people more quickly, and will not adversely affect highway traffic (though it will affect transit in the destination city due to the influx of workers). More advanced options will include ferry transport between cities with water access, as well as airports than can bring sims in globally.
Right now, however, as of 03/12/13, region play continues to be unstable and unpredictable. Monetary gifts between cities are being lost, even while still charging the city sending the gift. Cities are not being paid the appropriate monthly charges for providing power, water, and sewage. These issues are also affecting the transport of sims between cities for any reason, whether that is education, work, or tourism.
In short, it is difficult to answer this question right now. We know what the developers are intending to have happen, but we have yet to see it due to bugs and server strain. Once these issues stabilize, i will update this answer to something more concrete.