The Invasion: Your spouse regularly opens the bathroom door and saunters in when you’re going about your business.

The Defense: “Simply close the door with a ‘Let’s maintain the mystery, shall we, darling?’” says Marsh. Indeed, whether you are showering, are in mud-pack mode, or just want a few minutes to yourself, shutting the door will make a clear statement and may mean you don’t even have to say anything, agrees Peter Post, author of Essential Manners for Couples (Collins, $22, www.amazon.com).

But what if it’s a girlfriend who enjoys spending quality time in the bathroom, chatting between stalls or tagging along with you on trips to a teensy ladies’ room? You risk bruising her feelings if you flat-out ignore her. So if you must say something, again, try a joke. Molly Erdman, a comedian with the Second City troupe, in Chicago, suggests “I require full concentration for the task at hand.” Your friend will recognize that sometimes two is a crowd.

The Invasion: Your spouse and kids leave their things (toys! socks!) in your spaces.

The Defense: Think of the old dorm-room dirty-dishes trick: Plonking the offending plates on your roommate’s bed prompted a quick change in those housework-avoiding habits. The same technique can be used at home today — setting the sweat sock you found in your lingerie drawer back on top of your husband’s dresser, or removing your children’s colony of Incredibles figures from under your desk and resettling it elsewhere. The key, says Robert Sommer, professor emeritus of psychology at the University of California at Davis, is to make it obvious that you have deliberately moved the items from a place where they didn’t belong and weren’t welcome.

Prevent recurring clutter creep by making a clear and specific place for everything — a bowl for house keys, a separate bureau for your husband, a toy chest for the children — and label these areas if necessary, suggests Barry Izsak, the president of the National Association of Professional Organizers. Ultimately, though, says Erdman, it’s best to maintain some perspective. “While I don’t prefer that my husband keep his socks in my drawer,” she says, “I can calmly put them back where they belong knowing it would be much worse if he stashed them in the silverware drawer.”