If you need a costume that is simple to make and easy on the wallet, then consider Lisa Simpson. This precocious second grader is easily recognizable. and getting into character as her is lots of fun. Better yet, this costume is comfortable to wear, unlike others requiring masks and flowing capes.
Make a tube shaped dress out of red material. Sew elastic into the top to hold it on, and cut a jagged edge in the bottom of it. Lisa's dress isn't smooth and hemmed at the bottom so make it ragged looking for a more authentic costume.
String several wooden beads to make a necklace and spray paint them white. Similar to her mother Marge, Lisa Simpson always wears her trademark necklace.
Cover your face, arms, shoulders and legs with yellow body paint to get that famous Simpson flesh tone.
Find some red baby-doll-style shoes at a thrift store or garage sale. If you can't find that style shoe, red flats would do as well.
Make that trademark Lisa Simpson hair by spraying yellow temporary hair spray in your hair. When it dries, spike your hair so that it forms a row of peaks across your head. If your hair is too long to spike, buy an inexpensive wig at a thrift store and use the same process.
Carry a toy saxophone with you and get into character by telling folks about how much you love watching the "McNeil-Lehrer Report" on television and that you're a vegetarian.
Use the yellow hairspray even if your hair is blonde. The bright yellow color will add a nice effect to the costume. Study the Simpson family and really get in character. Talk about your underachieving older brother Bart and your wacky father Homer.