Keep a tight core and limbs, the ramp will try to pry you open when you launch so if you come off all loose and relaxed you'll be wacky wavy inflatable tube man in the air. Shin pressure, tight core, actively pull your arms in, and a slight pop will set you sailing smooth and clean, but a lot of it is just repetition. I haven't hit many large jumps lately, but I used to hit the pro line pretty often at stevens pass. Basically just tuck yourself into a ball in the air and it's less likely that an errant limb will start waving or pull you off balance, it will also set you up nicely for learning grabs.
When you get to really large jumps you have to recalibrate what is a comfortable landing zone. On jumps 15-30ft you generally don't want to land more than 5-10ft past the knuckle or else you get close to running out of landing. On 50ft jumps if you are only landing 5ft past the knuckle you effed up, cause that's not a good margin for error. You have to start getting comfy landing well past the knuckle and learn that it can actually feel good to go 20ft deep into the landing. It takes awhile to get comfortable with, because it's so different from hitting smaller jumps where sailing past the knuckle like that is a bad time.
There is no hard and fast rule to get you to judge the speed right and nail the landing perfect every time. I wish there was. Ultimately it's guesswork from watching other riders, a sense you cultivate over time, and the best way always is to get someone who knows the speed to tow you in. All that being said I still end up rolling down the windows often enough, it's just gonna happen sometimes.
