I would always use snap caps to dry fire my guns as well.
Zippy gave you good advice / and so did Bob on the tactical side ....
A trick I've used is to stand in a room where you have 10 or 12 feet to a wall - and face a light switch on that wall. The idea is to look at the switch, close your eyes, then mount the gun - open your eyes - and without moving, see if you are looking down the rib at the switch. Do it 20 or 30 times in a session / once or twice a day and you'll be surprised how smooth your mount gets.
Another trick - is to mount the gun in front of a full length mirror. Close your eyes / mount the gun / open your eyes and without moving your head - look at where the gun is / look at the angles of your elbows .... and groove the mount, so you have exactly the angle you want in your elbows.
I've also seen a drill where you mount a small flashlight in the lower barrel / turn it on - then mount and move the shotgun along the line where the wall and the ceiling come together - as you get to a corner, pull the trigger - and keep the gun moving as you follow thru. If that light is jumping all over - its a good indication your eyes are not focused on the target ( the crack ) and that you're going back to your barrel and stopping your swing ...
I would also suggest some DVD's - Sunrise Videos has a technology they call an eye-cam / and a lot of videos on sporting clays, Trap and Skeet - that are all very good. The website / has a "try it" button where you can see some of the stuff the videos present. But some real good stuff there from pros like Todd Bender, Bobby Fowler Jr, etc ....
http://www.sunrisevideo.com/