I've found l2tabu to be a very useful guide to avoiding bad habits. Yes, it's occasionally over-dramatic - one of its sections is entitled "Deadly Sins"! - but it does provide good reasons for all the taboos it lists.
But it does only cover sins of LaTeX coding, and not sins of bad typography. I tend to work on the principle that TeX and LaTeX were designed by people who know much more about typography than I ever will, so there's probably a good reason for the default settings. Thus, if in doubt, leave it alone.
That advice probably applies to the question of whether or not to create style files: unless you're creating a whole load of documents which you want to look identical in style, creating a style file only reduces the portability of your document, since you have to remember to include the extra file when you transfer it, etc., etc..
But, if you are writing style files, or class files (which are rather harder to get right!), Appendix A of the LaTeX Companion is exactly what you need, though if it gets complicated you might want to refer to the TeXbook when you need native TeX commands.
Finally, if you come to make pretty pictures and don't want to have to include EPS files and cart them around with the document, the LaTeX Graphics Companion tells you a million and one ways to get lovely diagrams with just a few lines of code; xy is particularly useful for commutative diagrams, though I mainly use pstricks for basically every kind of diagram anyone ever needs.