Dress Up Parties
The next time that you throw a birthday party for your child, invite all of his friends to come dressed up. If you have a themed party, such as princesses or cowboys, you can have the attendees dress up in a costume that goes with the theme. Just make sure that the invitation notes that all children should come in costume, and remember to keep a box of costumes on hand in case any child forgets and doesn't want to be left out. Try and think of costume ideas that go with the theme that you can suggest to other parents whose children are attending.
Thrift Store Dress Up
If your kids are getting sick of the same old dress up clothes, then try doing a thrift store dress up activity. Give each child $5 to spend, and head to the thrift store for new clothes for the dress up box. There, your children can find glamorous prom dresses, funny hats and gloves and even shoes that they can add to the box. For a few dollars you'll have hours of extra play, and the children will love being able to pick out their own items of clothing.
Tea Party Dress Up
Instead of having the kids play with the dress up box just to dress up, get the dress up box out and then announce to the kids that they are invited to a fancy tea party and must dress to impress. You can get in on the fun too, by dressing up in some nice clothes and baking a few treats for the tea (or juice) party. Give them a 20 minute time limit to get ready, and announce them formally as they come to the dining room for a fancy tea party. You can give the kids a character that they can try out, like a snobby rich girl or a prince. They'll love having an actual reason to get dressed up.