My personal fave is to make sure that lighting is low, with colored lights just bright enough to socialize without anyone bumping into things. I remove ALL traces of normal decorations - family pics, goofy/friendly/fun pictures, knickknacks, pretty things, trophies... and anything that can't be removed, becomes creepified.
I put up empty cobwebbed frames, or my collection of very old black and white photos from the early 1910-1920s. I have haunted mirrors (you make them using regular large frames, water, and spray on mirroring and then add in a spooky ghost image to "leak" through the mirrored effect). One really ornate frame I just spray painted gold, the interior where the image should have been was black, and then I used blood red paint and my finger to write the words "Help Me" and most all of my framed art is just slightly crooked.
I have an assortment of haunted house decor scattered about, and all furnishings are covered with white sheets, and curtains are switched out to torn, distressed gauze/cheesecloth or ornate Gothic fabrics. I have tons of skulls, pumpkins, vultures, crows, skeletons, coffins and other touches. Fancy draping and cloth over any table that looks too normal and I drape lights with shredded cheesecloth and hang spiders over them to look as if they were infested.
One of the other big things that really set the atmosphere - SOUND. Sure, have some music in the main area of the house especially if you have dancing but the halls and especially the bathroom should have some mood music - sobbing, sounds of echo-ing giggles, creepy whispering or moaning... I hide small speakers and mp3 players in the air vent in the bathroom and I have always done it up crazy since you have a captive audience in there. I keep the volume low enough that you can't hear it unless the door is shut, and depending on the rest of the theme, the bathroom has been covered floor to ceiling with black plastic and cut out magazine faces with their eyes scratched out (serial killer theme) and clown room (lots of bizarre circus music with laughing and crying) and skeletons in the bathtub and chalk markers all over the mirror...
Our dark hallways generally are great to just pose a standing figure in and NO LIGHTS so it's not obvious unless someone goes towards them... and tries to turn on the light. We switch the bulbs out so they are either flicker lights of dead bulbs (just unscrew enough not to work if you don't have a dead one handy).
I love things like haunted rocking chairs, and I have a moving eyes portrait too that I created with the help of one of the other forum members here (DaveInTheGrave) that is WONDERFUL as it looks as if someone is hiding behind it shifting their eyes back and forth...