I took a basic wedding dress (see my Princess Vespa cosplay for photos of the dress as it originally was) from the Salvation Army and sewed on sleeves, a "chestpiece" with a choker and cameo, and then I added silk flowers and some extra bustle.
Veil: I took a tiny tiara from Claire's and hand-sewed a huge train of bridal fabric to it. I then carefully arranged some paper roses (bridal section at Michael's) in the tiara, and then used a glue gun to create cobwebs on the flowers. I actually hand-ripped huge gashes and holes in the veil...to make it flutter better and look ghostly!
Bouquet: Hand-made from supplies at Michael's. Given that "old, dying flowers" look by dipping the silk flowers in a pot of coffee! I then took a glue gun and made "spiderweb strands" all over them. Looks great!
The glowing heart apparatus: I took a small 10-light strand of orangish Halloween lights with a battery pack (from Michael's) and gave them a thin coat of red acrylic paint...not to much, as I didn't want to dull the lights...just make them more red. I kept the battery pack in a plastic bag (protect from sweat) and the entire thing, lights and battery pack, fits in the bodice of the dress. I arranged the lights to sit above my left breast; it stays there due to the tightness and good fit of the dress itself.
Hasn't been entered in any contest, but it was a Disney staff favorite...as well as a guest favorite (they thought I worked there...)