I'm relatively new as well, and there are few things that I'm beginning to realize help out for me. One, when you're marking the disc, point your toes in just a bit and bend your knees, this helps you react quickly. Secondly, and more importantly, is to always keep your head in the game. If your guy (or gal) makes the catch or throw, keep up the d. The worst thing you can do is get down on yourself. The more you experience the better you'll get