Gymnastics!!! If your horse goes through happily, you can forget about trying to find a spot and steer and just jump. My favorite is 3-5 one-strides in a row with bounce poles in between. Like pole, bounce to jump, bounce over pole, bounce over jump, etc. Then you can concentrate on your jumping entirely. For my horse it's trot in to pole, 9ft, x-rail, 9ft, pole, 9ft, verticle, 10ft, pole, 10ft, oxer, 9ft, pole. Adjust up or down for your horse's stride, but you want to keep it as short, bouncy, easy distances. Large cross rails work well if your horse has a hard time staying straight.
For the roached back do the above exercise with a long crop/dressage whip/short broom stick held hooked through your elbows and behind your back. Or, just lock your hands behind your lower lower back
if you drop your stirrup and reins and go though you'll find the right rhythm and position after a couple a tries.
With a lower leg that slips back, I tie the stirrup to the girth with a single loop of baling twine. The loop should be pretty loose and allow the stirrup to hang in it's normal position and to move about 3 inches behind the girth. It never fails to amaze me and my students how quick this fix is. Once you feel where your leg is supposed to be in the air you'll be able to mimic it and gain a lot more security.
On the flat, I try to picture George Morris yelling at me and telling me all the things I'm doing wrong. Then tells me I don't know how to ride a horse, and he starts barking out commands: Counter canter, flying change, halt, back 3 steps, sitting trot, small circle, half-pass etc, etc. Which, of course, I execute flawlessly,
and he is very impressed. Goofy, but it works for me!