At the range is where it starts for me. Benchwork and testing hand loads will taught me a lot about my trigger. Holding your site picture and follow through are great habits formed in any position. However, I almost always finish a range session with off-hand shooting. I like multiple targets or aiming points. With a full mag(hunting cap) I like to shoot once at each aiming point. This can also be done from a different field position for each shot.
Practice prone, kneeling, sitting, and standing. If you're getting shooting sticks practice with them. I like a lot of the suggestions so far, and agree wholeheartedly about the required hunting accuracy. As for slings you can use any to improve your hold. While not as effective as the ching sling you can put your arm through the sling as it hangs from your rifle then moving your fore arm outward and back over the sling getting a wrap of sling around your arm. This will help you stabilize the rifle while moving your elbow down and against your side.
Any improvised rest in the field is great, but shoot shoot shoot(practice, practice, practice). Also one thing, you seem more confident off hand with your open sited SKS than your scoped Tikka. Try practicing with your scope on its lowest setting(what is that by the way?). High scope magnification amplifies any minor movement, shake, or tremor which can ruin your concentration on your sight picture.
Practicing off hand can be done with any rifle or firearm, I often practice with the 22. You could even practice with an air rifle, just focus on good habits that lead to better shooting. It's probably a good idea to practice with a scope as close to your Tikka's as possible. Of course if its affordable to practice with the Tikka go ahead. When I was shooting the most frequently me and my friends would shoot close to a brick of .22 per weekend and at least 20rds from our centerfires each. Try to shoot on a regular basis as well. The worst feeling I had was when I realized I had gotten rusty and I wasn't as good as I once was.
Popped this little 2pt with my kids from my knee at 110yds Oct 14th. He was in the trees and my only shot was the shoulder. The bullet went through and through exactly where I intended. Not an amazing shot, but I hit him right where I wanted to.
Also, find a mentor if you don't have one. You have an SKS, there are many friendly SKS shoots at rod and gun clubs. A great place to get advice and hone your skills.
Good Luck and Good Hunting,
Willy