I've never jumped a bike real far yet, but quads I have. This would apply to either, the first time hitting a large jump requires the following...
NO fear of what you are about to do/or at least very little
NO second thoughts half way on the approach
NO doubting yourself (this is a big one, got me hurt more than once), believe it or not, if you go at the jump knowing you can do it and you have it in the bag, it usually goes pretty well because subconscious experience takes over
And at least try and get a speed estimate/gear rpm range and so on if it's a really big one.
What a lot of people tell you, keep going farther and farther until you clear it... well, apparently they haven't jumped a double, a table top works well with this method, but a double... it will get you killed, it's either go or no go there
Goodluck, and have faith in yourself!