I can't stress enough how important it is to get all the grinding scratches out with the first grit you start with. If you're going up to 600 grit and finding scratches from lower grits, you're not looking close enough before moving to a higher grit and/or you're not changing directions with every grit! I know, I've been there many times! I use a piece of O1 as my backer when I sand. It leaves grind marks nice and sharp and you can really bear down with it. Look up the tutorial by Nick Wheeler on sanding. You'll get a lot of good ideas.
I also started using Gator belts to try and finish before sanding. When I started using Gator belts and RhynoWet paper, my sanding got a lot better in a lot less time!