1. Write your CSS mobile first
Write as though you are only targeting a mobile device.
2. Work in gradual stages
Once the mobile device looks good add styles in media queries for larger viewports in stages working your way to larger sizes.
3. Keep your media queries grouped by component and not in a separate section or stylesheet
If you make a code change 2 months in the future you will forget about the media query style in the separate stylesheet.
4. Limit the number of pictures
Mobile devices have slow connection speeds so limit the number of pictures. Mobile first pictures should be sized for mobile viewports and as you scale swap out for larger images. Thishelps a lot to get the <picture> element to work across browsers.
5. Test early and test often
I try and do each section or component at a time… if you wait until the end you will be hunting through tons of line of code.