There are some special things about training salespeople to become sales managers.
First, the skill sets are very different. While there is some overlap, most sales managers need to be able to step back and let others do the work. They also need to be comfortable with organizational mechanics and details, something that many fine salespeople simply don't pay much attention to.
Just to make things harder, often the very skills that made for top performance in sales are counterproductive in sales management. Persuasion, a key skill for salespeople, may need to take a back seat to training, which has a very different dynamic.
Also, in many organizations, salespeople can make more money without moving up. Alas, that is often not obvious until after accepting a promotion.
Now, how do you deal with these things and still come up with good sales managers? Start with selection.
The best salespeople are usually not the best candidates for sales management. They're far too likely to take over other people's sales work themselves and give short shrift to the training and developing aspects of the sales manager's job.
It's important to pick competent salespeople to promote, though, because they have to be street credible. So, look for competent sales people with good administrative skills. Ideally, you want folks who are "consciously competent," that is they're able to describe the sales process as well as simply do it effectively.
If possible, give folks an opportunity to try out the management role, perhaps by filling in for a vacationing manager in another area. Help them understand what the role entails so they can make an informed choice. Finally, give them good management training before they start working as sales managers. I also recommend giving folks the option to move back to sales within a few months if they find the manager's role isn't for them.