it's impossible sit straight and to move your bod forward without your seat tilting forward... follow his neck only with your hands, not your whole body. you have three standard positions in the canter; sitting, two point and half seat. what @ameliaduplooy is talking about is a half seat. but a half seat is to get off your horses back and give him some freedom, to slow him down you'd block with your seat. or if you need to give your horse more balance and you're in a half seat just open up in your upper body.
Well I usually stand upwards while I'm leaning forward, it helps me to have my horse more in control and also making him slower since he's so small and fast also an Arab. It helps me alot and I feel comfortable so. As long as you lift your bum upa little from the saddle it'll be fine.