Scott W. Ambler (I don’t think I need to introduce him, do I?) has published online many of the modeling style guidelines that he presented in the book The Elements of UML(TM) 2.0 Style
Instead of providing yet another resource on UML focused on explaining the language and its notation, it presents modeling guidelines (from layout guidelines to naming guidelines and to more general modeling advices coming from his own experience like “Indicate Visibility Only On Design Models”) to improve the quality and readability of your software diagrams.
It was time to have something like this. We have plenty of style guides for any programming language but this is the first one (AFAIK) for modeling languages (in fact, a google search retursn 170000 results for “programming style guidelines” and only 7000 for “modeling style guidelines”, and in 6999 of them modeling refers to mathematical and simulation models)