I found this video very helpful in using a sling and shooting from field positions:
http://www.youtube.com/watch?v=l-lzvjGP ... 84&index=2If i can't find or get to a good rest, i'll get down into the kneeling position as shown in the vid; if there's time, i'll get into the more stable kneel but if i've been spotted, i'll just drop to one knee with my sling tight. Trick is to 'point your stance' (even kneeling is a stance) at the target and not the rifle.
Under 50m i might shoot standing but not if much further than that, maybe out to 80m. I will shoot from kneeling out to 100m, maybe 150m but anything past that i'll take time to find a rest or somewhere to get prone, or pass on the shot.
Standing, i've found that keeping elbows into your centerline (close to body as you can; not chicken wings) helps a lot. Look at the target and raise the rifle to your eye. The target should be in the scope now. Breath out and lean into the rifle (no chicken wings). I find i reach the end of my breath as i lean in and am very stable and on target. As soon as i've reached the end of this movement i touch the shot off.
I'm not the most experienced shooter but have found since shooting like this (whist standing) my accuracy has gone through the roof and percentages way up. By the way, also practiced & taught kung fu for years so know a little bit about training, stance, stability and bio-mechanics.
Also, as mentioned by others, get your 22 out and shoot shoot shoot. I set up empty cans out of the recycling, 3 or 4 at various distances and practice repeating the above. Squeeze the trigger as soon as the sights hit the target. Practice 'stepping into' the shot as the rifle comes up while you have the target firmly in your view. What i mean by this is allow your body to align so that when the rifle comes up you are already mainly on target and not searching for aim point with the scope or open sights.
Hope that helps. It has done for me. Take or leave what ever you want from it all.
Cheers
Here's an article i also found very helpful:
http://www.chuckhawks.com/off-hand_shooting.htm