Relatives can sometimes pop out out of nowhere and it's unspoken that they will come to your house--and make you stress till your hair falls out. Having a system is easier than improvisation, and relatives will be amazed how prepared you really are.

[edit] Steps

Do a quick cleaning. Depending on how dirty your house is, it is relatively simple to do a quick cleaning throughout the house. If it's reeking with filth, you may be SOL but if you just have a little clutter and dust, pull out the broom and dustpan just a day before they arrive.
Adjust the settings in your home to make your relatives comfortable. If Aunt Mabel hates sunlight, draw the curtains and dim down the lights an hour or two before, so you can become accustomed to it. It's better to know what they like and dislike before they visit.
Keep games handy. Watching television and holding conversations can die out quickly, but if you have a game on hand it makes things lively and interesting. Card games are suggested, but board games are also very good choices. Remember, Aunt Mabel most likely does NOT want to play Halo.
Hide and stuff. If you have offensive things, especially when inviting young children or elderly folks over, lying around the house, stuff them into cupboards no one will use or roll up sexed up posters. Aunt Mabel will not appreciate discovering your pornography collection in the magazine rack while she's going to the bathroom.
Keep a spare bedroom or couch. If your relatives are staying for the night, it would be good to keep a set of extra blankets or pillows and allow them to sleep on the couch. When push comes to shove, offer the relatives your own sleeping quarters, but don't come right out and suggest it--you've got to sleep somewhere, too!

[edit] Tips

Hide porn.
Watch your mouth.
Delete crude or offensive text messages--because Aunt Mabel might just be suspicious of your social activities.
Same goes with your browser history. Delete it ALL. Aunt Mabel is nosy.
Burn any pictures of Aunt Mabel that you doodled on. The inheritance money will go straight out the window if she finds the moustache.

[edit] Warnings

Don't have a whole bunch of relatives over at once, unless it's a party or social gathering of that sort.
Don't invite creepy relatives over that molest children.
Don't have an affair with your husband/wife's relatives. Things get messy.
Aunt Mabel is off limits. Sorry, boys.