Use a dark purple or black liner on your lips! Buff them really well with a little sugar, put on some lip balm, then the liner (as a lipstick) and then top it with a little more balm.
Also, straighten your hair or put it into two braids, Wednesday Addams-style. If you don't have a long black shirt to wear underneath of your clothing, cut a hole into the crotch of some old blank tights, cut out the feet, and wear it like a ballet shrug -->
Because y'all know goths don't show bare skin. If you really want to get into it, mix some talcum powder into your foundation to make it lighter than normal.
I will sadly, sadly add that this comes from experience. Not because I went to a costume party, but because I was an idiot 15-year-old
Fishnets, Fishnets, Fishnets!
On your arms, on your legs, a tube around your neck as a choker... heck, even try a piece of fishnet stocking as a veil attached to a little black hat, for that "Lolita" mystique. Or pin a piece to your hair, along with a plastic flower (black, red, or purple) as a fascinator.
Cleopatra-esque winged eyeliner. Cheap bright red rouge can be a great Goth eyeshadow, blended with a little bit of Vaseline for staying power.
If you can find it, white eyeliner looks incredibly spooky lining your lower lids and the crease, down to your nose. It makes for an ethereal cat-eye look. If you have anything with an ankh on it, that'll help lots.
If not, anything stars or moons for jewelry.
Black nails. VERY important to have black nails. Sharpie works in a pinch if you don't have polish.
And, of course, the best accessory is a permanent puckered scowl!
Call yourself Morgana Ravenmoth for added flair.
(I used to be a Dark Wave kid... I think I still have my black velvet hooded walking cloak packed away somewhere, shame I can't lend it to you for the night. )