(MONEY Magazine) – If you are dealing with a full-service brokerage firm, you're paying about $80 in commissions to buy or sell 100 shares of a $30 stock. That's roughly 50% more than you would pay a discount broker, so you should expect some additional service. One thing you deserve is the best possible price, whether you're trading 100 shares or several thousand. So ask your broker these two questions before you put in the order:

1 Are you going to use the exchange that offers me the best chance for getting the sweetest deal? Your broker is obliged by state securities laws to get the best execution for you that he or she can. To do this, the broker must canvass the various exchanges to find the lowest posted offer (if you're buying) or highest bid (if you're selling) and route your order to that exchange. But then, sometimes, the trader processing your order can negotiate an even better price. Here's how it works.

Suppose when you put in an order to buy IBM, the best offer on any of the six national stock exchanges is $110.25, and the best bid by buyers is $110. That makes the spread between the bid and the ask 25' a share. Sometimes a broker in the crowd is holding an order that has not been posted--say, a sale offer at $110.13. If the trader with your purchase order finds such a broker, you'll get a better deal, by 12' a share. Of course, your trader need not stop there. Depending on your order's size, the trader's clout and whether the exchange facilitates negotiations between buying and selling traders, your agent can haggle with the seller to get even more for you. Let's say they agree on $110.10. Bingo! You've just bought IBM 15' cheaper than the lowest asking price. Not bad.

Only about 20% of all trades get such bonuses, called price improvement in Wall Street parlance. While price improvement is a great deal for you, brokers don't always route orders to the exchange where it's most likely to happen. Why? A dealer in an exchange who wants to build his trading volume might pay brokers in dollars and discount services to route orders to him. Every time you make a trade, you should tell your broker to send it where you have the best chance of getting the sweetest deal. If you notice that your broker repeatedly uses only some of the national exchanges, you may want to give your business to someone who will check all of them for you.

2 Will my limit order get the maximum exposure to the market? Limit orders, which are instructions you give your broker to buy or sell when a stock hits a stated price, account for 35% of all trades. Assume you've told your broker: "Buy Coca-Cola when it hits $47." When Coke gets close--say, the best bid is $46.88 and the best offer is $47.13--you want everyone to know you're a buyer at $47. An impatient seller might be tempted to let you have it at your price rather than hold out for $47.13. But unless your broker exposes your bid when it's the best around, you may not be able to make that trade. Exchanges handle limit orders differently, so insist that your broker send yours to the place where it will be matched quickest. --M.F.