Of the 4 conventional positions, standing is the least stable and therefore, the most difficult.
NPA is critical.
Your position should utilize your skeletal structure as much as possible, as compared to muscling the gun.
As a result of the front sight always being in motion, it is imperative that you execute the shot properly. Mounting the gun, sight alignment, trigger control, breathing, follow through, etc. All are critical in becoming a good off hand shooter for slow fire target shooting. Rapid fire action shooting is different, but the basics still apply.
I am going out today to practice, you're welcome to join me if you'd like.
An extremely important point! Don't fight it! Relax! Don't assume you'll shoulder a rifle and it will stay rock solid on target like shooting prone with a bipod. The front sight will move. Put all of your other shooting skills/diciplines (npa, breathing, trigger control, follow-thru, etc.) together and 'use the force'. If'n ya try to anchor that sight on the x-ring, you'll wear yourself out. It's even worse with a scope with crosshairs!
ol' poke
If'n ya try to anchor that sight on the x-ring, you'll wear yourself out.