Ask if your guests have special dietary needs and clearly communicate special needs to your caterer.
Order a wide variety to meet the needs of vegetarians, vegans, etc., and those who are more health-conscious.
If your audience dominates a specific culture or religion, be sure to serve cuisine appropriate for them.
Planning a lunch meeting? Opt for an easy-to-eat, fortifying lunch that is pre-prepared. If the meal is to offer a reprieve from a day-long meeting, consider a more playful meal that allows guests to customize choices (i.e. sundae bar).
If coordinating a stand-up reception, steer clear of anything with bones, dipping sauces, shells, etc.
Remember to assess your space and time constraints and order food that complements each.
Depending upon the nature of your event, you may have 5 to 15 percent attrition and can almost guarantee a few walk-ups. Discuss with your caterer the overage percent they prepare.
Ask for a tasting so you can see the portions of food. For example, if you are hosting a lunch buffet and the sandwiches are cut in half, it is likely that most guests may only take a half of a sandwich.
Have you accounted for staff or volunteers who should also be fed? Be sure to include staff or volunteers in your meal count.