Details on how NOT to purchase a firearm.

Quoted from ZeroSumHappiness

First, when the clerk pulls out the gun it's only polite to pull out your own. If you don't have one then you should pull out a toy gun. This shows the clerk that you're also a strong creature capable of defending yourself.

At this point the clerk will ask you what you are doing. This is normal, it's part of the dance we call "making a straw purchase." That's what it's called when buying from a gun store, a "straw purchase," because back in the day gun stocks were all made from light pine wood and looked like straw. Over time, as they've aged with the oils from being handled, they've turned into beautiful brown oak.

The proper response to the question of what you're doing is, "Doesn't matter, just do your job and hand over the gun." This informs the clerk that you are knowledgeable about your constitutional rights, such as the right to be left alone to go about your business. Gun store clerks respect such assertive answers.

At this point your gun should be in your right hand and you should point it at the clerk and take the gun he's holding with your left hand. Make sure that during this your gun is firmly trained on the clerk. This shows you know how to use a gun effectively to disarm someone, a useful trait as a gun owner.

Now that you have his gun you should holster your own weapon and get to checking the new gun's function. First is to make sure it would fire. The correct procedure depends on what type of gun it is. For example, with a bolt action you should quickly cycle the bolt and the pull the trigger, dry firing upon whatever target is closest. Often this will be the clerk first. Repeat this action, changing targets quickly and efficiently, continuing to prove that the gun functions properly.

After you have tested that the gun functions correctly you need to fill out the 4473 form. The clerk will help you with this. Make sure that no matter what you do you answer the questions in the way that will get you the gun. This is because if you don't lie but are a prohibited person, such as a convicted felon, you will be denied your gun. So lie if necessary.

After you've paid and taken possession of your new gun be sure to test it out with live ammunition. Street signs are excellent targets to practice your shooting on the move from your car or truck.

tl;dr -- Always be polite. Don't remove a gun from your own holster unless you first check with the clerk. Check that it's empty before doing anything. Ask before dry firing, ask if you don't know a safe direction to dry fire in the store -- many stores have targets set up for dry firing. Thank the clerk afterward. Gun stores are one of the few things where it truly should be safety first.