Keep It Finger-Friendly. Unless it's a sit-down dinner, finger food is the best way to go. Look for simple, elegant ways to present food, whether it's making crispy cups out of parmesan cheese or using endive leaves as spoons.
Make It Interactive. Assemble-your-own foods — such as tacos or summer rolls — give your guests a built-in conversation starter.
Keep the Crowd Moving. If you're not doing all finger food, be sure to arrange a buffet table with an eye to traffic flow. Make it easy for your guests to serve themselves: pre-slice larger cuts of meat; wrap utensils in napkins and place them at the end of the table; keep condiments all in one place with spoons. Clearly label foods to help alleviate any confusion.
Plan Ahead. No one wants a frazzled host. Do all of your prep work in advance (cutting crudités, assembling sandwiches, etc.) and keep whatever extra food you have on hand in marked containers in the fridge. When you have to refill the buffet and your kitchen is crowded (which it will be), it'll be much easier if you're prepared. Also, consider asking your guests to bring something, like beverages, desserts or appetizers.
Anticipate Ice Needs. If your fridge doesn't have an icemaker, buy ice. You'll almost always need more than you think.
Serve a Festive Dessert. Birthday cakes are always appropriate and appreciated. Just don't forget the candles.