My 14 year old boy and I run MX, and we find that every track has it own differences and it takes time to learn each one.
Get the bike as wide as the rules permit and a +3, (+ more is OK too) swing arm that will allow you to set the jumps up fine.
Learn one obstacle at a time, get it right then move on to the next.
Do not be pressured into taking chances by the other guys who have got it right.
Take your time, get it right and you will be in front of them in no time.
Perfect the tricks on how to keep the nose of the bike at the right level, if you are nose down on jumps, blip the throttle, that will pick it up.
Nose up, grab the clutch in and hit the back brake to bring it down.
When going into them burms, if they have them, dot the back break,( well thats the way I do it, as I am prone to coming in too fast!) to get the ass out and committ to the corner long before you get there.
It takes guts and determination to do it, but if you are already sideways, others find it hard to go under or around you as you are almost twice the width.
You possibly wont need to use gears 5 or 6 with your 14/40 set up so drop the front sprocket to a 12, and maybe take the rear up a couple. This will give you an extra gear to play with and also give you advantage over the hole shots, as you can get off in second and you get that extra get up and go coming out of corners.
Choose the tyres for the surface, Kendas, and some others, come with the option of rotating the tyre to suit a hard or soft surface.
Bit of extra toe out, and get the camber adjusted to get them tops of the wheels in.
Tyre pressures are very important, experiment and run to what suits your weicht and riding style, I like no more than 4psi in the front and no more than 6 in the rear.
The way the bike rides and handles depends on the shocks you use, if you use stock shocks be prepared for a very difficult ride, they do not work well for MX.
As Awk says if the other guys are getting it right, look and learn, there are no prizes for taking chances, unless you rate a ride in an ambulance a good trade off.