First of all, sales people are there to help you and it's important to remember what is in their control and what isn't. Treat them with respect and be kind, using please and thank you. Sales people who are treated kindly and respectfully, will almost always treat you the same way. And at your favorite stores, a sale person can be your greatest asset in notifying you of sales or new merchandise!
Do not hold salespeople accountable for things outside of their control. If the dress you want is out of stock, or you can't get a refund you think you deserve, ask to speak with a manger who will likely be able to best help you with your problem. Likewise if you aren't being treated with respect by a salesperson or they aren't helping you with something that is in their field of expertise, feel free to mention this a manager.