Blacklight Primer
What you can do with blacklights is limited only by your imagination.  They are relatively cheap, as are blacklight reactive materials.  This page gives you some pointers as starting points.  There are other examples among the decorations featured elsewhere on this site.  See the "Fun with Blacklights" for starters.

Types of blacklights NOTE: THERE IS NO SUCH THING AS AN INCANDESCENT BLACKLIGHT.  I don't know why these are even sold anymore.  These can commonly be found at Walmart and other commercial retailers, even at Halloween stores.  Even though they are usually advertised as "party lights," the implication in the color and design of the packaging is that they can be used to fluoresce whatever they shine on.  THEY WON'T.  DON'T BUY THEM!

Things that fluoresce under blacklights:

Anything fluorescent

Anything that glows in the dark Unexpectedly: Whites (sometimes; this is hard to predict): Use the empirical approach:
I picked up this small, hand-held blacklight off eBay.  They also sell these with pet urine cleaning products as urine stain detectors.  This is the perfect size to take out with you or to explore around the house and see what lights up.  I've found a lot of things that unexpectedly fluoresced.

Techniques for incorporating blacklights into decorations Chromadepth
Combine blacklight effects with chromadepth 3D glasses.  You can read more about it here, but the short version is that things appear three-dimensional based on their colors.  Blue appears farthest in the background, while red appears closest.  As such, if you use a blue background and color things in red, green, etc., the image will appear to jump out at you.

I have seen this effect used in several haunted houses.  For example, painted walls, especially with speckling (e.g., paint splatter) in different colors*.  The effect is even more pronounced when it is done on real objects.  All of a sudden, things that are already three-dimensional will appear to have additional dimensions.  It's a real head-trip because your brain can't process exactly what's going on visually.  One example that really made an impression on me was a skeleton that was painted fluorescent green with fluorescent red blood stains across it.  The red appeared to jump out in addition to the blacklights making the colors really pop to begin with.

*In most cases these were fluorescent colors under blacklights.  However, any basic pallet of blue, green, yellow, orange, and/or red will work.  There are numerous examples of pictures and videos that will work.  Run an image search and/or check YouTube.

The glasses can be found in bulk on eBay or even at party supply stores in some cases (I found some at Party City).

Further reading

Copyright 2011 the Ale[x]orcist.