Keep your heels down and sit up straight with your shoulders back, that way if the horse stops your heels will be anchored into the stirrups to stop you from flying forward and with your shoulders back it will stop you from leaning forward too much or falling forward
Make sure not to get ahead of the horse. When you go over the jump, your position moves forward slightly, if you do this before you reach the jump then you are already off balance and much more likely to fall. Stay balanced on the horses back and line others said have strong legs and core. Working without stirrups will help with this.