In the weeks before your cocktail party, do a little bit of research and find the very best cheese store in your area. Your local farmer's market will often give you the opportunity to speak directly with the cheesemaker, who will ensure that the products are in the best possible condition.
Talk to the person behind the cheese counter. An enthusiastic cheesemonger can be a trusted resource. The cheesemonger should be able to make suggestions of cheeses that will complement each other on a cheese plate, and possibly the establishment will have a few pre-selected combinations for entertaining.
When cheesemonger is unavailable, check the labels, especially on fresh cheeses, to make sure that the product is well within its expiration date. For larger cheeses that are cut down to smaller amounts, the store label should also include an expiration date near the weight and price amounts. If a cheese is reduced in price for quick sale, it is generally not a bargain.
Choose 3 to 5 cheeses, depending upon the size of your party. Aim for variety, hard and soft, mild and strong, cheeses that appear different on the outside and on the inside, different milk types, blue veins or not. Purchase a total quantity of cheese equal to four to six ounces per person depending upon what else you may be serving with the cheese.
Make sure the cheese looks fresh. The cheeses should be characteristic of their style, with an interior that is free of cracks, discoloration, and mold (unless it is a blue cheese). Note that natural rind and other artisan cheeses may have rustic appearance, which is one of their attributes. When possible, taste the cheese before you buy it. If you're unable to taste the cheese but want to give it a try, but the smallest amount possible.
Because of the wide variety of dietary concerns and restrictions, check labels for the type of milk (cow, goat, sheep) from which the cheese was made, whether it is a pasteurized or raw milk, and whether it uses animal, vegetal or microbial rennet. If the label doesn't say, then ask. A good cheesemonger will be able to tell you and be happy to steer you in the right direction, especially if religious, dietary or animal rights concerns govern your diet.