Do you mark or hi-lite your patterns or can you follow them without any markings? Do you make copies to mark up, or do you mark originals?
Despite the fact that I do counted cross stitch, I don't like to count much. So, for complicated patterns--and what makes a pattern complicated can vary--I do highlight the areas I have done. I usually make a copy to do that, though. If I wanted to do the pattern again, I would need a clean copy!
So what makes a pattern complicated? Confetti stitches usually require highlighting. This helps avoid confusion, "am I stitching that little heart symbol by the dots or that one?" Right now, I'm working on Angel Stitchin's tree from the (2005) JCS Ornament issue, because so many of those lines of stitching look just like the last one, I'm highlighting. Since I'm stitching on the train, I'm highlighting a little less frequently. Let's face it, the pattern's not that complicated--you know, if I actually bothered to count.
What I find fascinating are the pictures people show on their blogs that have stitching done in little 10x10 squares. I'm intrigued by it, since it's just about the opposite of the way I stitch. They have all these right angles and square edges in their incomplete pictures. My stitching is more freespirited, with blobs of color running in all different directions (I prefer to stitch one color at a time, within certain parameters). But I guess that's why I need to highlight.