Plato's Closet: This is a no brainer, you can sell your old clothes and make enough money to buy an entire outfit within an hour! The staff in Lawrence is always helpful and genuinely wants to help you.
Arizona Trading Company: More affordable than Plato's Closet but not quite as nice, it's a incredibly fun shop and there are some really fun pieces here.
Goodwill, Salvation Army: Always a good place to start with your thrift shopping!
Ditto: although they carry a better diverse selection than Plato's Closet they are less strict about what clothing they buy so often their clothing is dated and in lack-luster shape. None the less, there are always some gems here.