Just as it is important to have costumes, it is important to have props. Props— short for "properties"— are those bits and pieces that clutter up your rooms, the things you hold, the tools you use. While you can mime a lot of things, you have to have really good actors to do a haunted house without props.
Naturally enough, most people will look to Halloween stores for prop pieces. In general, this is not the best idea as they can be cheesy and expensive. Go ahead and get a few set pieces from the holiday store, but make or find the rest.
A surprisingly good reference for prop spectaculars is Martha Stewart. MS Living has published ideas from your dry ice cauldron to mock hanging moss made from black trashbags. There's step-by-step instructions on the many uses of cheesecloth. Cheesecloth is inexpensive and versatile. Further prop instructions can be found online at Ghosts of Halloween. More cheesecloth!
But you don't always have to start from scratch. Look around your house. Especially look around your tool shed. Take a look at the dollar bin of an antique store. Find something strange and wonderful and disturbing, and figure out how to work it in. Even a perfectly ordinary cellphone can be good if you rig it to ring its absurdly cheery little ring... from the bottom of a well.
There is one problem with props, however, and that is they will get damaged. Don't use your mint-condition heirloom as a prop, and likewise don't use anything that is apt to break. If using an electric tool of any description, find some way to render it harmless. A chainsaw, for instance, is perfectly safe once it's had its chain removed— there's nothing to cut or tear, and in dim light your guests won't notice.
But the most important thing about props is "don't be boring." Anyone can get the crawling hand from the holiday store. It takes imagination to make some old paper and photographs scary, but it's worth the extra effort.
Next up: Positioning