I have been really working on my jumping ability for a decade. I've ridden with some very good riders who have offered tips and tricks for getting airborne. The closest I came was riding with Rhys (now of Maydena Bike Park in Tassie, but then living out of a van near Whistler BC) and somehow feeling imbued with a sliver of his jumping prowess (he does no-handers and backflips with the ease I ride off a curb). But two days later I had lost the mojo.
Then I ran into a video on YouTube from Lee McCormack (of LeeLikesBikes fame, purveyor of Cycling Kung Fu, teacher of many skill-deficient professional MTB racers, and co-author of MTB skills books with Brian Lopes). Lee talked about rowing up the face of the jump, and for me that was the epiphany. I suspect it didn't change very much of what I was already doing, but the rowing action demands that you remain centred on the bike (you cannot do it out of position). And voila, I could consistently clear smaller jumps. My main issue with bigger jumps is risk. Falling hurts a lot. And can take months to recover from. I don't want that time off the bike, or the rehab, so I'm cautious.

As a person's jumping experience grows, they can "know" the appropriate speed window for a new jump they haven't done before. But that is hard-won experience. There has to be a first jumper, but if that's the jump builder, they should know exactly what they had in mind... Everyone else can learn a jump by following someone who has done it previously.
I was all set to get towed into some jumps at Thredbo this summer by a young guy I'd been riding with a bit - but then he got a jump wrong and smashed his humerus to pieces. I knew I could do those Thredbo jumps so I tried two of them, and was able to do them easily on several occasions. Phew! Woohoo!! I left two more untested for next season (I was cautious!).

I realise how visual I am with jumps from my recent experience with a small one on Trebuchet at Stromlo. Just before the over/under intersection of the DH tracks and Trebuchet, there is a small rock lip on the right. It was surrounded by weeds that grew up in Spring nearly obscuring the rock. I couldn't easily hit it. As soon as Iconic Trails cleared the weeds away, it was trival to take off. Small change, big impact on my head.

I have taught a fair number of people to use better jumping technique in their riding, but what was working for others wasn't producing the consistency I was looking for in myself. Lee's rowing thing did the trick. Incidentally, he also teaches rowing and anti-rowing for pump track progression, but I can't get the un-row down. My own method works pretty well so I just go with that.

You might want to check out Lee on YouTube, or hit me up for a lesson if your own jumping isn't up to scratch.