At home, over a LENGTHY time period, with the help of your trainer, by acquiring a deeper and more secure seat. Do not even WORRY about it for the show. All you can do at this point is make it worse. Things are how they are.
And what are some faults and disqualifications in Hunter Equitation on flat and over fences?
What you'd expect. On the flat, it is bad to do things that are bad in any other rail class -- fail to take up requested gait, break out of gait, wrong lead, having your horse get into an altercation with another horse (this is the rider's fault). Also posting on the wrong diagonal. Beyond that, it is judged on the basic overall correctness of your riding, which is not something you can alter for the show ring.
Over fences, obviously you have to go over them, without stopping or running out, without knocking them down either, in the correct order
It is best to be able to get correct leads around turns and the # strides in lines that the course is designed for -- but neither of these things should concern you at this point in the slightest. Neither should posing for the judge or trying to disguise riding faults. Just shoot for getting around the course cleanly and smoothly. Again, in schooling shows this is sometimes quite adequate for a ribbon no matter what your equitation is like <g>
DO NOT TRY TO CHANGE YOUR RIDING FOR A SHOW. Not only is it utterly pointless, it is usually counterproductive and makes you do *worse*. Schooling at home -- i.e. the other 99.5% of your life -- is where you should be working on slippy knees or wrong diagonals or things like that. With the aid of an instructor.
Just stop worryin', already, and have FUN, ok? HAVE FUN. H A V E F U N . You know? Seriously
And then, afterwards, can I make a suggestion? You would avoid almost all these questions by going and WATCHING shows. Seriously. As a spectator. You can learn a tremendous lot that way. It is usually a good idea to carefully WATCH a division or 'type of show' a number of times before you even contemplate entering yourself in it. By that point, pretty much all of the questions you're asking us, you'll already have answered... and more!
Pat