I guess just practice, practice, practice
DING DING DING... we have a winner! FirstFreedom hit the nail on the head: you gotta get out there and practice.
However, reducing your wobble area doesn't require live-fire practice and can easily be accomplished indoors. I practice in the garage using a black paster on the wall as an aiming black.
A fellow highpower shooter showed me a drill that I use nowadays. It has greatly helped my offhand shooting. My AR comes in at nearly 17 pounds, so it really is a workout.
- Get on
all your gear as if you're shooting in a match: coat, glove, stool, hat, sweatbands, boots, etc.
- Establish your NPA and shoulder your rifle.
- Dry-fire the shot and hold that position for 1 minute, paying attention to your wobble.
- Lower the rifle and rest for 1 minute.
- Repeat for 20 simulated shots.
I practice this drill once a week at home indoors... usually Wed evenings after work. Sunday mornings are spent at the range getting some live-fire practice in. When I have a match coming up, I'll spend the Mon, Wed and Fri before snapping.
Whatever/however you decide to practice...just be sure to stick with it and you'll see an improvement.