The Brian Lopes book is good, but I find that many people can read what to do and yet never actually apply it properly while on the bike.
Getting a good instructor to work with you means you will get the feedback you require and they will be able to spot what you are doing right/wrong and get you to do the right stuff.
You can learn on your own, but there is likely to be a whole lot more trial and error, it will take longer and you might crash a bit more while doing it.
Bike setup can make a bit of difference, but is not the key. Where you are looking definitely helps (if you keep your eyes on the take-off ramp the whole way through you will probably bin it).
Learning to pump correctly makes a huge difference. Learning where you need your weight on the bike makes a huge difference. Being able to relax makes a huge difference. Finding an instructor that can teach you these things is the quickest way to make a difference.
If going it alone, find yourself a set of jumps that you can progress on and start small. If you are not confident of making the jump you will tense up and do things wrong. Once you have small mastered, step up to the next one etc
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