When I first started I had a couple of friends who would come over while I decorated. It was nice to have sounding boards and they saved me from over-decorating a few cakes. Now that I work past midnight sometimes, they are not as willing to come over. LOL.
I find that sketching my design out ahead of time really helps keep me focused. They don't have to be stellar works of art (the sketches), just a rough idea of what I'm going to do so I make sure I have all my details lined up.
I agree with Eileen, I find the sketches to be extremely helpful. Seems like if I draw out what I want I end up with a descent looking cake if I just go by whats in my head without a sketch thats when I usually end up not as happy with my cake.
I always sketch my cakes out in advance... then make all the decorations in advance. Then I follow the sketch. I know where you are coming from though... I think we are all guilty of the same thing... it is really easy to end up with something different from your vision when you don't have a plan of attack!