Begin by setting the mood outside your abode. Plant a few gravestones in your yard for ambiance, and switch outdoor light bulbs with colored bulbs to create a glowing red or blue effect. Add to the dramatic effects by having guests enter through a side door or through the garage rather than the front door.
Some of the best haunted houses are confined to a few rooms or a basement -- decorate with skeletons, cobwebs, mirrors and fog. It's a quality, not quantity scare. Keep lighting dim (an eerie glow is creepier than pitch blackness). Play scary sounds or music in the background -- a well-placed scream can make the night.
Remember to keep in mind the ages of your partygoers when you design your haunted house. Go all out with gore and ghouls for teens and adults but keep things lighter for kids -- you're aiming for spooky, not nightmares.