Please don't rip signs off shelves or fixtures to "prove" that something is supposed to be on sale. An employee needs to verify that the price you cite actually refers to the item you're buying, and it's much easier for them to walk back with you than to replace the sign after you've torn it off.
Please, please do not say "you look too young to work here!" to the cashier. If she's working here, she isn't.
Do not expect employees to watch your children while you shop.
Not every employee in the store is able to do the job you want done. Employees folding clothes or stocking shelves might not be trained on register, and even if they are they might get in trouble were their numbers to show up in the register system. Cashiers may also not be allowed to leave their stations to help you find something (but of course, they should promptly find you someone who can!).
As annoying as it is, cashiers are often required to ask customers a set - sometimes an excessively long set - of questions. Not asking for zip codes, customer service surveys, charity donations, and especially credit card sign-ups may lose the cashier his job. Cashiers understand all too well that this can be incredibly frustrating for the customer, but there's nothing they can do about it. If you find it to be a hassle, most cashiers will gladly tell you where to direct your complaints.***
Keep in mind that cashiers may work in every department of a large store and thus may not know everything about Product X. Don't be nasty or imply that the cashier doesn't know how to do his job because he says "I don't know, but let me find you someone who does." I was often assigned to run register in a commission area of the department store where I worked, and anything I knew about the products in the area (quite a lot by the time I was done, but not everything) was purely bonus and not required by my job description.
This should go without saying, but sadly it can't: never make assumptions about the cashier's knowledge or skill set based on his or her gender!
If an employee doesn't understand what you want or doesn't know how to help you, never assume that she's an idiot. She might be new.
In many stores you are not allowed to use someone else's credit card. Period. It is usually a violation of both the terms and conditions of the card and store policy. Many cashiers disregard this policy for the sake of speed and customer satisfaction, but just because "that lady who was here yesterday accepted the card" does not mean your current cashier can't refuse to do so. In these days of identity theft, do you really want someone to be able to walk into a store and buy things on your dime?
Remember the golden rule of being a customer: you can ALWAYS catch more flies with honey than you can with vinegar!
***If you only remember one thing from this post, remember this. I can't tell you how much verbal abuse I and my fellow cashiers had to endure because corporate didn't have a clue how annoying customers find endless questioning. I counted once and I think I determined that I was required to ask customers up to 11 questions before they could pay for their items. And if I skipped a single one I'd get in trouble.