Training. If your pony is moving when you go to mount, you need to get him some ground work - not learn to jump on a moving pony. Problems need to be corrected, not worked around.
Start by asking your pony to stand while you mount. If he moves forward when you go to step in the saddle - stop, and back up our pony quickly a few steps then try to mount again. It may take 3 min or an hour but sooner or latter, your pony will associate moving while you try to mount with having to work (horses don't particularly like moving backwards). Once you are mounted, it is very important that you stand there for a minute or so. Your pony also needs to learn that mounting does not mean forward movement - which is what probably started the habit in the first place.
There is never a quick fix to undo habits. It takes time and patients.