It's been a (far-fetched, possibly) goal of mine to some day write a math Textbook. I've been thinking about writing this question for a while, but reading an exceedingly mediocre text on Mathematical Modeling has finally provoked me. In several paragraphs, I have already spotted numerous changes I would have made to the book (not the least of which is publishing somewhere besides Elsevier, haha); so it got me thinking about mathematical exposition.
At this point I'd really just like writing advice. I've seen some articles e.g. (A Guide to Writing Mathematics and seen Serre's "Writing Mathematics Badly," and I know some of the "basic" authors that are celebrated expositors (Halmos, Spivak, Rudin...)
But I'm asking for you to share the benefits of your experience: Unforgivable mistakes you've seen made (repeatedly), great expositions you've read, or any other sort of advice you have (apart from 'just do it!' I am!).
Certainly I'm lucky to have this great website to practice on and see a vast number of others' style.
Lastly: I am not entirely sure if this question is appropriate, or if it is too broad or vague.