Remember that whether you are having cocktails before dinner or your cocktail hour is the main event, don't set up this area like a dining room. You want your guest moving and mingling, not planted in a seat.
Consider placing bars and food stations around the room instead of all together. This forces guests to move around the room and encourages them to meet and greet the other guests.
Remember to place bars where they are visible to your guests, but avoid putting them near entrances and/or exits. Bars tend to have lines and you don't want your guests piling up at the door and preventing other guests from being able to safely enter and exit your event.
Be sure to communicate with your guests. If you are planning on having a cash bar make sure that your guests know this before they arrive so that they show up with cash in hand.
Avoid setting up bars and buffets near the dance floor or music. Make sure that guests can move freely around these areas with their hands full (plates, glasses, etc).
When setting up your cocktail tables, be sure to provide seating for approximately one thrid of your guests. Cocktail tables should be small and seat no more than four people. You may also consider using bar-height pedestal tables. Guests can lean on them and put their plates or drinks on them. Many rental companies even offer bases that turn 30-inch-high tables into high tops.