Best thing you can do is practice. I got some 12ga snap caps and practiced using them over-and-over, and over, and over and over... until it became pretty easy.
Under stress it does get more interesting, especially when you add walking or running into the mix. One thing I found that REALLY helped was joining in on some local action shotgun matches, 3-gun would work well too. Granted, nobody is shooting back but the stress of the buzzer and trying to get a decent time cranks up the urgency and exposes weaknesses.