Yep, a neck strap is a nice way to really save your horse’s mouth.
I tend to grab some mane with one of my index fingers (before each fence), it gives me a bit to hold onto!
Also, as an eventer, we have been taught to ride XC with your stirrups “home”. That is, putting more of your foot through the stirrup. In the ring, I ride with my baby toe at the outside branch, “outside” I bring the stirrup further back, half on, half behind the ball of my foot.
Practice really jamming your heel down at the girth, with your stirrups “home”. It will help give you a secure leg in the case of trouble!
And yes, no perching before the fence. Not a dressage seat, but eventers sit (while keeping a bit of forward angle with your upper body) 4-5 strides before the fence. Get your bottom in the saddle, legs tight at the girth, grab a little of that mane, and HOLD ON.
Sitting before the fence will also help your horse wait for the base of the jump. Don’t want crazy long spots over solid obstacles, and usually the other horses will provide plenty of motivation to gallop over the fence.
Have fun!