The key to success at double & triple combinations is for the rider to have a clear plan! You must know precisely what the distance is between the elements, & also what your horse's natural stride length is. Then you have to be able to analyze the combination to make a game plan. Learn how here!
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.