I totally agree with @skiep on looking ahead. You spend too much time focusing on the bouys. Keeping your head up keeps your hips up. As soon as you know you will make it look at the next one - especially on 2-4-6.
As for staying stacked try this little tip. Hold your arms out to the side, parallel with the ground. Picture a line that extends forever. As you are skiing, starting with your pull out for the gate, don't let the line touch the water - ever! Pretty impossible to do, but if you are trying it will help the stack position.