Even if you and everyone with whom you share a bathroom abide by these rules, it's unlikely to ensure a spotless environment. No one is infallible. We are human, we have hair. A stray strand here or there is no reason to sound the alarm and release the hounds. Keep your expectations manageable. A generally clean bathroom with minimal clogging, a toilet seat that's normally down and a shower that doesn't double as a garbage disposal are good places to start
If you feel that you must leave a note for the offenders, or hold a dorm or roommate meeting about the general nastiness, keep it civil and try to avoid scolding or finger-pointing. No one likes to feel that their mother just walked into the room. Focus on what needs to change rather than making personal attacks and you just might see a turn for the better.