The Other Couple
Double date only with couples that you know well or that you feel confident of establishing a deeper relationship with. If you and your boyfriend are into sports, for example, don't double date with an artsy couple. Choose a couple with which you share some common ground, but avoid coworkers or childhood friends; the two of you can end up talking about the office or reminiscing while your significant others sit there with nothing to say. Similarly, don't go out with newlyweds or a couple that only one of you likes.
Choosing an Activity
What you do on a double date depends on how well you know the other couple. If you're not close, go with the standard dinner and a movie. If you know each other well, try more adventurous activities or restaurants. Never make a decision without consulting the other couple. Before making concrete plans, consider everyone's budget, dietary restrictions and time. Plan a few days in advance in case you need to make reservations somewhere or swap ideas for a while.
Keep the conversation open to everyone. Don't lapse into private chitchat with your significant other or start a girls-only conversation that excludes the guys. Talk about things that everyone can discuss, but avoid sensitive issues such as politics or religion. Avoid bringing up memories that only two or three of you can reminisce about to avoid making anyone feel left out or awkward. Don't argue with your significant other; if you do, excuse yourselves and go outside to work it out. Hashing it out publicly will make everyone uncomfortable.
Clearly answer the "Who pays?" question before the date. Be on time, or call if you can't avoid being late. Sit next to your significant other so that you can easily address both members of the other couple. Divide your attention equally between the other guy and girl, but don't neglect your own significant other. Don't flirt with the other guy's girlfriend, even if it's meant to be funny. Don't drink too much; alcohol often eliminates inhibitions, and you might regret what you said or did the next day. End the date at an appropriate time, and never make the other couple feel guilty for going home earlier than you'd like.