Nothing is scarier than when a young, unassuming woman dresses up as a corpse bride. It subverts the beauty that we expect to find in a woman who is at the prime of her youth, and presents us with a chilling vision of death.
What would a bride be without her wedding dress? Your ghoulish figure should be adorned in the finest garments you can find at the party store. If you want something a bit more authentic, a trip to a thrift shop may be in order; you can often find wedding dresses and other formal wear for quite cheap.
Get an old, worn wedding dress - the kind a ghost lurking in the attic of an haunted house would wear. At any rate, try to find something inexpensive, because the look will include adding a few rips and maybe some blood stains.
If you want to go all out, have some wounds showing through the tears in the dress or some other indication of how you met your demise; some inexpensive latex prosthetics can make for quite the convincingly gruesome wound.
If you're a crazed ghost who was scorned on her wedding night by a murderous husband, your hair is unlikely to be pristine. Make things messy and unkempt, comb your locks up the wrong way and build something of a rat's nest if it doesn't bother you to walk around that way all night.
Alternatively, if you're afraid of split ends on your real hair, or your hair is not long enough to give the kind of effect that you're looking for, you can always wear a crazy wig (or wear a nice one and mess it up a bit). Don't forget to top it all off with a veil.
This is what will really lend authenticity to your ghostly form. You're going to at least need a few wedding-related items; for example, throw together a bouquet of dead roses to represent the love that could never be - extra points if the roses are black, like your cold, dead heart.
If you're a psycho corpse bride, you may want to throw in an ax or a rusty dagger; these can be found in plastic form at most Halloween stores. If you maintain that you were the victim all along, perhaps you can drag around the instrument of your death, such as a noose.
All in all, a corpse bride costume is a great classic that you can bring out year after year, since it never really goes out of style. Just be sure to add some creepy make-up to your face and maybe even your hands to make your ghoulish outfit all the more convincing.