I found that the best way to get warmed up is to start your jumpshooting from about 2 feet out. Well not JUMPshooting, but more of a free throw. You just want to make sure you get the same release every time. So when you start from really close, you can get your rhythm going. I usually shoot about 5 2-footers, then start moving further out. Get to the free throw line and shoot about 10 free throws. This usually works for me. If you have your rhythm going, you'll know right away. A big thing is to shoot like your shooting in a game. Sometimes during warm-ups I'll take lazy jumpers. But by game time, I'm usually good to go.
I'm just like you because I don't always get the same release. I have to find what's working that day. My shooting form changes day-to-day. It's a very slight change, but it can the difference between a good shooting day and a bad one.
You want to make sure you are balanced. I love shooting one-legged fadeaways (Dirk style...try it...it's actually pretty easy), but the key is to have balance in your jumpshot. When you do one or two dribble pull-ups, you want to make sure you're jumping off of two feet to get that elevation and balance in your jumper.
Another thing is to make sure your shooting elbow is not out, you want you elbow to be straight and you want to make an L-shape with your shooting arm. You don't want to extend your arm too far back, but you also don't want to not extend enough. Usually, it's a little less than a 90 degree angle.
But for me, I think the biggest thing is to use your legs in your jumper. Can't stress that enough. Sometimes, when my legs get tired, I start shooting short jumpers. Then to prevent that, I try to put more power in the jumper. It doesn't work. At that point, you're just aiming and not actually shooting with a solid follow-through.
So yeah, balance, elbow in, legs in your jumper, and of course, eyes on the target.
It sucks because I know I have the ability to be a very consistent shooter, but sometimes that ever-so-slight change in the shooting form throws me off. I heard you're supposed to shoot a jumper the same way 100 times to build muscle memory. Try that I guess.