When your horse has the required amount of muscle and strength the jog will slow on it's own.
I partially disagree with this. Yes, the horse has to be fit, because a slow gait as required in WP is pretty darn hard for a horse - but you don't see racing thoroughbreds jogging around do you?
There is a deal of training that goes on. What you could do though, which is what I'm doing with a woman I'm giving lessons to, is work lots and lots of circles in the middle. Tiny figure eights, get her moving off your legs. The circles thus collect her, and she has to slow down essentially because of the small circles, and when you feel her drop a gear, let her move out straight.
As soon as you feel her speed up in even just one step, start circling again, changing directions a lot. Eventually, you'll get more consistency, and she'll jog for longer periods of time.
*edit: For this to work properly, you also have to make certain she keeps her shoulder up. If she drops her shoulder, she's not going to get collected, and she's not getting off your leg.