No advice on how to train, since my pups aren't allowed off leash in unfenced areas (way too high of prey drive), but here is a link to a thread on mtbr where people posted about training their dogs to run with them on trail rides. It's good reading, and great pictures of cute dogs!! There are links within the thread that are worth clicking on. There are posts, of course, of how you shouldn't do it, just look past those and try to find the useful information you need.

http://forums.mtbr.com/showthread.ph...highlight=dogs

This link is one that is part of the one above, but there are a lot of cool pics
http://forums.mtbr.com/showthread.ph...ght=trail+dogs

Hope it helps!