Invitation: Using a pattern in the shape of a dress or suit, cut costume-type fabric in the pattern shape. (Fabrics could be polka-dot as for a clown, black satin as for a witch or vampire, or white for a ghost.) Write all of the party information on the fabric (use white ink on black paper if necessary) and mail.
Decorating: Visit discount and secondhand shops to find funny or scary vintage clothing. This can be period clothing or just mismatched items. Stuff the clothing with rags, fiberfill, or towels and prop in chairs or on the porch. Find an inexpensive mask to place on the top.
For a fun lighting idea, buy several large three-wick candles and eyeglass/nose costume accessories. Pin the glasses in place on the candles to emphasize the party theme.
Craft/Game: Divide the guests into teams. Give each team one pair of scissors, fabric scraps, and masking tape. Time the teams to see which team comes up with the best costume in the least amount of time.
Games: Divide the guests into two teams and have each team make up a list of movie characters along with what they wore in their particular roles. Have opposite team members try to guess the movie and film star by hearing only what they wore.