Consider a lightweight cardigan your all-season layering piece. When the weather's warm but unpredictable, it's light and breathable enough to prevent you from overheating during the day and yet cozy enough to keep you (or your date) warm when the sun or temperature dips. It's a simple way to add a layer without too much bulk. Look for one in a neutral shade and let it class up a pair of brightly colored chinos, worn-out jeans or cut-off shorts. Summer weight wool is a great option, but we lean toward cotton or linen simply because you can throw them in the wash when they get dirty and let it air dry.