1) Introduce yourself, you should know each other by first name. Try to use his name a few times in conversation. Let him know a little about what you collect. If knows what you're looking for, he can help direct you to what he has.
2) Make it obvious that you're not a thief. If you have a coat, take it off so there's no pocket games. Keep the stuff you looking at right on the counter in plain view. Don't be reaching in your pockets or doing anything suspicious. Unfortunately coins are pretty easy to steal and dealers lose lots of money to scumbags. A good relationship comes with mutual trust. My favorite shop owner would often leave me all alone while he went to the bathroom or to the storage area to get stuff for me....complete and total TRUST is important.
3) Buy stuff! Everytime you go in if possible, buy something even if it's just supplies or a $5 coin. Make yourself a regular, paying customer and you'll get special deals and perks. Never buy anything big right away, it's best to become a customer first and pay asking prices. When you're ready to spend bigger $$$, then you can negotiate as a "regular".
4) Bring your own magnifier and price list. It's absolutely fine and shows you're savy collector and serious buyer. Don't expect to pay CDN bid prices! They have expenses and usually sell above CDN ask in my experience. Of course, a lot depends on the coin, some coins just command more $$$.
5) Don't fight his grade not matter how wrong it is. Of course, coin prices depend a lot on grade, but it's insulting to the dealer to question his grade. I've found 99% of them get mad when you try to downgrade them. It's just better to pass on a coin when you think the grade is wrong. The good thing, this works both ways! I've seen a good number of undergraded coins and snagged some awesome deals! Whatever you do, don't tell them they undergraded. LOLOLOLOL Just shutup and say you'll take it.
Most dealers try to be a "jack of all trades" and know every coin series....well....very few of them are experts in all series and they make mistakes in their haste to resell. So happens I'm pretty darn good with Lincolns and I can capitalize when they under grade/under price. It doens't bother me at all because I know for sure they won't sell a coin for less than they have in it. They're making money from me....just a little less than they could have if they knew Lincolns well enough. LOL
6) HAVE FUN! Looking at and buying coins shouldn't be all business. Once you establish a friendship with your dealer it can be fun to just go in the shop and hangout for an hour or two. I often bring my favorite guy a bottle of water and a candy bar or something. He works long hours and really appreciates when I bring him stuff for the stomach.
It's really amazing how much you can learn by hanging out at the shop, observing what's going on and looking at stuff.... even outside of what you collect. My dealer is partners with 4 other guys that visit often and they are a trove of experience and knowledge. Over the years I've learned SOOOOOO much from all of these guys just by hanging around the shop. It's like a free university of coin dealing/collecting! I consider every trip a learning experience.
I'm so close with my main guy he digs stuff out of the safe to show me, like a $100,000 Rolex or all the returned coins from NGC/PCGS that were bagged. That's actually by far my favorite part....we routinely discuss bagged coins and I look over them in detail to figure out why. It's also cool when he pulls out a big dollar key dates that he just got back from being slabbed. He lets me look at all those! VERY, VERY COOL!