Mentoring Programs

I have been a part of mentoring programs at two different schools. I don't think either of the two worked the way they were intended. What do you think are the most important things to keep a mentoring program beneficial. The major complaints in our school were, lack of meeting time between mentor and mentee and location within the building. I think a good mentoring program could be a great asset but it seems that it is harder to put together than people think.