Clean and readable code, being consistent with your color & design choices and not mixing a lot of different things together because you can = good hud & allows for easier customization.
Using all the hud techniques that are available to you as a hud dev to make positioning stuff relative to each other easier, full panel boxes and removing hud elements or lines of code from your files that aren't used/hidden anyway and adding comments to your files so you know which things do what and are customized are good practices.