• Always, always throw out empty bottles and cans...or nearly empty bottles. There were many times when someone would come back from a trip to Target armed with new toiletries only to have to spend an hour yelling, "WHOSE BOTTLE OF SHAMPOO IS THIS? CAN I THROW IT OUT?! IT'S BEEN HERE FOREVER!"
• This one's more for roommates: If you have nice products that cost a pretty penny, keep them in your room unless you don't mind sharing.
• Clean the drain after you're done showering (especially if you've got long hair). Wipe any toothpaste splatters and loose hairs from the sink/mirror area (especially if you're a guy who uses an electric shaver. As one friend put it, "Clean up the effing man beard clippings!") With that in mind, it may be a good idea to get a plastic cover for your toothbrush.
• Set up a cleaning schedule so that the bathroom gets cleaned once a week. That way, it never gets too awful...unless your roommate or significant other is a bit lax on the cleaning. In that case, consider the option of hiring a cleaning service to do the dirty work (and splitting the cost).
• Limit your bathroom time, especially in the morning when everyone has to go to work and/or school. Probably not the best time to sit on the can and finish Tolstoy.
Got your own tips on surviving a shared bathroom? Post it in the comments!