In addition to standard letter-writing dos and don'ts, there are a number of basic guidelines that apply specifically to most situations related to the writing of letters of reference. These are usually more "situational" than "how-to" in nature.
These reference letter guidelines are important to both note and apply, since writing letters of reference is always a somewhat tricky and delicate matter. That's because they almost always affect the reputation and future of the writer or that of another person.