After extensive research and personal experience, I have found the best time to buy:
January: boots, cocktail dresses, hats, scarves

, European designer lines, housewares
February/March: winter sweaters, resort clothes, designer items
April: spring gear, tights, socks, sheets
May: bathing suits, beach gear
June: sunglasses, handbags
July/August: sheets, bedding, summer gear (i.e. shorts, flip flops, tank tops, short sleeved shirts, skirts, sandals), winter coats
September: denim (of anykind), designer items
October: rain gear (trench coats), women's suits, winter pants, fall clothes
November/December: leather gloves, cashmere scarves, tableware
Notice that most of the items are at the beginning or end of a season. Stores will push items right after arriving or if they have lingered in the store for longer than 4 weeks. To get the best price you almost always shop in the off season or end of season, but you run the risk of not having your size or the color.
research through personal experience, msn.com, budget fashionista, smart money.com , city chic by nina willdorf