Here's what our Code of Ethics says about that issue:
"Standard of Practice 12-7
Only REALTORS® who participated in the transaction as the listing broker or cooperating broker (selling broker) may claim to have “sold” the property. Prior to closing, a cooperating broker may post a “sold” sign only with the consent of the listing broker. (Amended 1/96)"
Most MLS regulations govern this practice as well; you need to check with yours before doing it if you're the selling broker.
"Just Listed" and "Just Sold" post cards are huge for me in getting business from many areas. If you sell 2 or 3 houses in an area that would be 4 to 6 post cards saying you were successful in that neighborhood.
http://www.ExpressCopy.com is an excellent post card resouce. They will print color on 2 sides, address them, and mail them (including first class postage) for only 45 cents each.