another trick (at least for me) is to apply a bit of technique I picked up shooting spring piston airguns, don't "death grip" your rifle, keep the muscles in your hands relaxed. I also keep my thumb laying alongside the wrist instead of wrapping it over the top, if I do jerk the trigger or tense my hand a bit this keeps my thumb from twisting the stock and pulling the shot to the right.
Chris
We have a young guy on our range who started out great then deteriorated quite rapidly as he developed his flinch. I knew his mother taught a yoga class so I told him to ask her to teach him how to relax under tension. He did as I suggested and his shooting improved outa sight.
It's all about the rhythm. His, and mine is:
start with the rifle at the waist, don;t take your eye off the target.
RELAX, take a full but not big breath and shoulder the rifle at the same time.
As you find the sight picture you are letting your breath out slowly and steadily. RELAX.
Relaxing while in tension is something that has to be learned. Holding our whole body in tension seems the natural thing to do.
As soon as the sight picture is good stay relaxed and fire, do not hesitate.
Nothing but the finger moves. The recoil will push your shoulder back. Let it, offer no resistance.
Throughout all the above your only focus is on the target and the front sight, stay focused through the shot.
Once practiced this process gets my shots off in around three to four seconds.
Miss fires during practice can be very valuable. If you are doing it right you should still be on target after the gun failed to fire.
Ignore the flash, ignore the flash, ignore the flash.
A good way to test yourself for flinch is to get someone else to prime, or not prime your firearm while you don't watch. You literally don't know whether, or not the rifle is going to fire. If there is a flinch it will be obvious.
Do not feel bad about flinching, if that is what you are doing, just about every shooter has had to deal with it. It's just part of the game.