Exercise usually helps. You can do leg workouts, and if you're riding, no stirrup work does wonders. As for landing, make sure you have a solid leg over the jump, keep your body solid, land in your feet, and try to get back to your normal riding position after you land. If you've got another jump following up, especially if you're doing grid work, if you're still leaning on the horse's neck and they happen to spook, it could be the difference in staying on or falling off.
Lots of trotting, jumping position and no stirrup work!
Plenty of hacking up hills builds muscle
practice sitting in 2 point or half seat on your horse. And when landing try locking your elbows on your horses neck to brace yourself.