Don't aim the gun at the salesperson or any other customer. If you feel the need to look down the sights, do it off in a corner after you've made sure you're not going to be having someone staring down the barrel of a gun. I HATE when I go to a shop or a show and I'm swept by some moron!
Don't take the word of the salesperson as gospel. Many have personal preferences that are not based on fact, and many are told what gun to push be it because they are told to do so by management, or if they sell enough of a certain product they can win cash or prizes. Go with what gun you feel is right for you, not on what the person behind the counter tells you is the right gun for you.
Don't fall for the whole "Stopping power" crap. If you're new to guns, don't be pressured into buying a .45 or a .40 over a 9mm because the you hear from some gun rag or a sales person that the .45 has more stopping power. There are plenty of dead people that will argue that the 9mm has plenty of stopping power. Again, go with what you like.
Do rent guns if you have a chance and shoot the ones that feel good in your hand.
Do shop around and check prices on the internet before you buy. Even after FFL fees, you may come out ahead.
Ask if they have a cash price and/or make sure the price they have on the tag is not a cash price. One of my local shops adds 3% to the price of every gun they have if you use credit as opposed to cash or ATM.
I probably could think of more, but my kids are using me as a jungle gym right now and I'm a bit distracted, so I'll let others add on.