Set Up in Front, Not Behind
For so many years I can't remember, I always thought the best way to hide was to sit behind something. I went out of my way to find low brush or even clear a path through brush so I could set up behind it and not be seen by animals coming in and still have a path to shoot through. Did it work? Yep, but more often than not, the only way I was going to get a shot was to either stand and shoot over the brush or move out to the side and shoot around it. And in so doing, the movement was always detected by whatever it was coming in.
I have since learned to make my ambush from in front. I like to find some bushes or some trees that I can snuggle up into with a wide open view in front. One thing I am always looking for is enough foliage behind me to completely block out all light coming through which might silhouette me. With the backdrop and some good camo, I can actually move more than when I was trying to stand or sneak out to the side. I have an open shooting lane in all directions and my movement, if slow, is less detectable than you might think.
I sit with my shotgun/rifle in a ready-to-shoot position so it takes little or no movement to have it up to my shoulder. And for those times that things happen so quickly that you don't have time to get the gun into position, I've found that being in the open with no obstructions in front lets you snap the gun up and get a shot off before the animal/bird has a chance to react.
I take a small folding stool to keep my behind up off the ground and give myself a little more stability. I set it up in front of a huge tree or a batch of bushes and I'm comfortable for an extended period of time. Setting up behind bushes and not having a backrest is killer on the back and you'll tend to move much more quickly. Always set up as comfortable as possible so you aren't constantly moving to relieve the pain. I always make sure I have a backrest and a place to dig my heels in. I not only want to be comfortable, I want to be in a good shooting stance when the moment of truth arrives.
Set up in front, get as comfortable as possible, have your gun in a ready position and sit still.