Look for sales, lower end stores that still carry trendy items. Pick out some shirts that suit your style. Maybe you want to add a red shirt to go with a beige suit. Or maybe you want a multi colored tee shirt to go with your black skirt. These are the pieces that can pull everything together and give your outfits a finished look.
Sweater sets are also great.
They come in many different styles and fabrics, from silk to cotton, and are the most extremely versitile. Tee shirts are also great. You can get tee shirts in rayon, silk, or basic cotton. They do wonders for a wardrobe. The right tee shirt can dress up an outfit or play and down and make it more casual.