When doing the straddle your legs are placed similarly to the position someone riding a horse has. Thighs out to the side and lower legs mostly vertical. You should neither be arching or de-arching. Relax your hip and that should do the trick. Keep your feet positioned straight forward as this will give you a better range in movement.
Exit Jumping out facing the tail of the plane is usually the easiest.
Jump out in the straddle position and lean back so your head is facing the front of the plane. Get your body horizontal as quickly as possible. That's only meant as an example to give you the visual impression of what it looks like, but if you want to try it make sure you're wearing a helmet and some kind of back protection. When exiting the plane this way you can use your hands to figure out when you get the right position. Hold your arms down your side with your palms facing backwards.