great casual shoe

And please don't forget about a great casual shoe because nothing kills a chics sporty casual attire like a pair of white running shoes. Stick with a pair of boots or sandals, etc. You probably won't get stopped by the "bad wardrobe police" with boots and they go with almost all casual clothing.
Use casual accessories and handbags -- especially when it comes to casaul wear for women. No two worlds are more distant from one another than the classic look and womens casual attire. Please don't try to mix a gold and diamond watch with a sweatshirt unless you're Queen Latifah. Try to stay away from mixing styles.
Keep your casual clothes in good shape -- don't be lazy if they need to be ironed then iron them, if they need to be cleaned then clean them, you get the point.