Strip the label from a bottle of Bacardi. Put red food coloring in there with the contents. Make a new label that reads nothing other than REDRUM.
And serve at body-temperature 
Fooooood-
Borscht looks like cream of “big purple dinosaur” soup.
Deviled eyeballs - Boil eggs. Crack and then dye them with red tea or food coloring with vinegar. Cut in half and make deviled eggs. Put a slice of pimento on each one for a pupil.
Pizza faces - Decorate English muffins with cheese, sauce and meat slices to make scary faces.
Kids party cookbooks are full of scary edibles, like appetizers poked into a baked potato to look like a porcupine, frozen bread dough shaped into snakes and critters, ice cream cone witches and weird cakes.
If you have an outdoor fire, kind of fun to do if you can, having s’mores and telling scary stories just go together.
Meat/jello salads made into a round gelled salad always freak me out. Of course, anything with tofu scares my guests.
Decorations-
You can find bubble makers for $20 - $30.
Bartman decorated his office last year like the gate to hell. (He makes a realistic looking devil - just add horns.) We put black marble contact paper on uniform size boxes and stacked them up to make walls. We made paper stalactites to hang up and scattered gargoyles around.
A friend covered his ceiling last year with aluminum foil and had dance lights reflecting off it.
I love dead roses in vases, it looks so cheery.
Decorate the yard with wood or cardboard tombstones. Put names of guests on them with weird ways they died.
Make tons of construction paper bats and hang them with sewing pins from the ceiling.
Cover helium filled balloons with drapes of crepe paper or very light fabric to make ghosts.
Party favors make fun decorations. My sister had her 18th b’day a while ago and I got kids’ favors as a joke. People were fighting over the hinged snakes. I suggest those, adults seem to covet them. Don’t count on having any stick around after the party, though.
I love halloween!
This year, one of the silly things I want to do is make a gingerbread haunted house like this: http://www.geocities.com/margosgingerbread/ghauhou.jpg
There’s always room for Jell-o. Especially if you can find one of those nifty brain molds. Mmmm… jell-o brain.
Aside from that… tell us more about the party. How many people? How many will be willing to help you with preparations? Are you looking for activities, or is drinking and hot-tubbing going to be the order of the day? Can you scare these people? Do you want to? How badly?
Did you know that, without a chain, a chainsaw is almost harmless, but very noisy?
I knew this link would come in handy someday:
http://www.fabulousfoods.com/recipes/dessert/cakes/kittylittercake.html
Bon Appetit!
A merkin, of course!
For the tombstones, of course, you should make the death date Halloween. Try to make most of the causes of death plausible for a party–drowned in a hot tub, food poisoning, maybe electrocution.
If you can set up two TV/VCR pairs in the same room, run “Nosferatu” on one and “Shadow of the Vampire” on the other (muted). Try to get someone to spend part of the evening as a “head in a jar” (head stuck up through a table into a box or open jar), or as a vampire, making disparaging comments about the latter.
Try to make jello-shots in nasty shapes–worms are particularly good.
The idea of having a fortune-teller of some kind is a good one. Too many people know a little about the Tarot to get away with faking it–if you can’t find someone who knows how to read those, try runes. It’s easier to fake. Crystals are even easier, but a good crystal ball is expensive.
Cobwebs–not strung all over the place (that’s cheesy), but in a few strategic locations. Also, the bagged webs that the party store sell is cheesy. I use a webshooter–it’s a weird contraption that consists of a PVC cement can mounted on a small fan, which is in turn chucked into a drill. The cement can has one pinhole poked in it over each fan blade, and you fill it with something like cold vulcanizing fluid–that site has both the gadget premade (and overpriced) and good fluid. You just hold down the trigger on the drill and neat, realistic-looking webs float down over your target. I’d drape them over the bathroom doors, for starters.