I do great at twenty yards. Thirty yards I would say I open up to about three inches or so and at forty I will also have a few fliers too. I do not know if it is good or not, but my best off hand shots are ones that are done right when I pull up, like a couple seconds after finding my mark. This is total free hand standing. Seems like the longer I look through any scope, gun or bow I start to wander, and it gets to the point where I have to pull down, breath and then pull back up. If I just pull up, aim and shoot quickly then I am OK. Now when I bench shoot I can do the procedure of breathing and all and have no problem. When I hunt most of the time I am in my tree stand and all my shots have been using my knee as a rest to steady my shots with.

I hear that hotburn. The longer I stare the harder it gets. Anyone ever try one of those horton knee pods? The steddy eddy is nice. I do practice a lot of free hand though.

that's why trigger pull is so important, imho. As you try to maintain your sight picture, you keep applying pressure to the trigger, Let up when pic is bad, and increase when it's good, hopefully the two will merge for an accurate shot. Some of my best shots offhand were when I thought I missed by a bunch. There may be a substitute for practice, but I haven't found it yet, lol.

Practice good shooting dicipline listed abover and practice, practice, practice. I practice every day, mostly offhand, at 20,30 and 40 (occasionally 50 yards just for kicks) I plan to use a rest since all my stands have them. But, that way I know that I can shoot no matter what the situation presents.
But the biggest factor to improving shooting is a bow with a good trigger. Should break at no more than 3 lbs and minimum creep. Believe it or not flinching may be causing your flyers. And a poor trigger just increases the likelyhood of a flinch. We know the x bow won't buck but it's only human nature to tense up at the shot. Watch for it, then go back to good shooting dicipline.

wrap the sling around your wrist/arm....

someone mentioned a tripod, i use a stick i cut from tree for my muzzleloader...

as you get older, you cant hold like you used too.....

crossbow is not light..

I do all my shooting offhand and how I hold depends on if I am standing or sitting,and yes the sling is a very good support.....

I spent a few hours shooting at 3D targets yesterday and even the shots I would consider flyers if I was shooting spotswould have dropped any deer within sight of my stand. I am a pretty fair shot but balance in even a slight breeze can be tough. Those limbs are like sails..

I have also started trying to shoot ONE arrow at a time. Flinging 50 just leads to fatigue and not improvment and after all it's the first arrow that counts..
My tree stand practice is usually off my knee anyway along with a few over the shouder shots. I shot a buck last yr. @ 31yds off my knee and missed my POA by a few inches. Still doulbe lunged him..