Here is what I have done:
~I sell too large items (if they are nice, name brand like Gap, Old Navy, Lane Bryant, etc.) on eBay. I then use the money in my Paypal account to buy an item or two in a smaller size.
~I shop CLEARANCE racks at stores. You can often get a new top at Wal-Mart or Target on clearance for $5 or $7...
~I buy things that are very basic that can mix and match easily in what size you need now: a basic pair of jeans, a pair of black pants, a white or black t-shirt or blouse...you can have 5-6 items that if they can mix and match-can create a lot of different outfits. Spend the "money" on accessories that will fit over the long haul-such as a great pair of shoes, a great necklace, or a nice colorful purse-or a great new haircut. A simple pair of jeans and a black top look very hip if you have a great bag, or a great new cut & color to go with it.
~Shop Goodwill and garage sales. I check garage sale ads and see which ones have the items I want...I have gotten great items from Goodwill as well-Old Navy, Gap, Abercrombie and Fitch, Nike, Reebok, Lands End...you just have to "dig" a little.