Confidence is crucial in Eventing. If you or your horse don't believe you can do something, you most likely won't be able to do it, even if you are capable. A solid foundation with good basics builds confidence levels. Many riders are pleasantly surprised to find out they CAN do it.
To make your seat stronger when seated in the saddle, think of using your core muscles in a downward fashion.
Horses need to be honestly connected and on the aids before they can stretch properly over their entire topline. Without that connection from behind, the stretch will be the horse's neck only, which is far less productive than a real stretch over the back.
In the sport of Eventing, ensuring that our horses are truly fit for our level of competition is one of the best ways to prevent unnecessary injuries.
Always use both reins together to steer, especially when jumping. Using one rein only turns the horse's nose - but using both reins turn the horse at the shoulders, which means his body will more accurately follow your chosen line.