A box full of costumes is an important part of a house in which kids live, all year long. Why is it so good? - Because it allows for self expression, it develops the imagination, it lets the kids assume different roles in invented games, and it's sheer fun!
How can you fill such a box without spending much money buying costumes (which are bound to bore the kids after a short while anyway)?
The box: could be any kind of box, preferably with a lid, prefereably relatively wide and short (to allow for easy searching inside). A cardboard box can easily fit, provided it is strong enough: paste the folded parts to keep them in place, wrap the box nicely from the outside (it would be nice to use a lovely plastic gift-bag, for instance, or a colorful paper sheet covered with transparent cellophane).
Clothes: do not throw away any old clothes that have anything special about them. For instance: shiny clothes; metallic clothes; especially big clothes (that can be cut and used for sewing new clothes); leopard-like clothes; clothes with extraordinary designs, etc. Gloves, for instance, can almost always serve for this or that costume. Long skirts, especially wide ones, can turn into gowns (worn over the head, like ponchos). Baby pants (those with feet) are perfect for wearing on the head, as rabbit ears. Some of these clothes can be used just as they are - see for yourselves: put them in the box, and see your kids give them imaginative interpretations. On the other hand, there's a chance they will just take up space in the box.
Just take up space: In this case, you may start cutting (and not necessarily sew anything anew). For instance, red, black and white fabrics can make excellent pirate bandanas, if you cut out a wide triangle with two equal sides (an isosceles triangle). Sleeves of various colors can become various animals' tales: a grey sleeve, for instance, can be filled with fabric leftovers, well-creased newspaper or cotton wool, and become an elephant's tail (or rather a mouse tail...). For the elephant you might add a black pompom at the end. Attaching the tail to the body is easy if you sew (or glue) a long, narrow strip of fabric to the tail, and tie the strip around the waist.
Accessories: hats, kerchiefs, belts, old jewelry bits - anything that triggers the imagination. For instance, a simple old belt became a Bob the Builder belt once we attached to it some plastic tools and a rolled building plan.
Crowns: use construction paper, cardboard, or any other easily-manageable material to cut a strip with a zigzag shape at the top, and decorate with bright and shiny colors, glitter glue, shiny objects (such as various sequins). You may want to add pompoms at the crown's tips.
Makeup: some toy stores sell cheap lipsticks in various delighting colors. If your children have no allergies, they can use these to color their whole face easily and cheaply. While you're at it, you may want to buy some cheap nail polish, especially glitter shades, and use them for craft: much cheaper than other colors, widely varied, easy to work with (comes with a built-in brush!), and highly durable.
What other accessories do you keep in your dress-up box?