Number one thing:
NEVER MAKE A FABRICATOR DO MATH! This is the number one frustration and mistake/part scrapping thing I see on all drawings.
Give dimensions that you can actually measure not goofy BS like through a solid part that you can't put a tape through.
Correct tolerances. Does a weldment really need to be +/-.005? This kind of stuff wastes so much time, especially on poorly engineered stainless sheet parts.
Designing things that are impossible to do well without an inordinate amount of pre/post-processing.
Design parts where you can get your. MIG gun or TIG torch into whatever it is that needs to be welded.
Learn to bolt/rivet/spotweld/epoxy stuff together if you want fast throughput with minimal distortion.
I could probably rant for hours about this.