I have a number of checklists I use, from deployment to form design, but the practice that has had the biggest impact for me is creating on-the-fly checklists while I'm coding. I often set out to do one thing and realize there are four or five other things that need to be implemented or changed. I used to try to keep track of everything in my head, but when your mental todo list starts looking like a complicated digraph, that just isn't effective. Now I don't code without a scratch pad by my side.