Practice a wide-eyed expression in the mirror and raise your eyebrows. Hold the expression for a few seconds to mimic real astonishment.
Open your mouth, imitating a "jaw dropping" look and bring one or both hands up to your face. Either put your fingers in front of your mouth, or place one hand on each side of your face.
Utter the first few words of an exclamation but don't complete the statement. "What the...?" or "Oh my good..." will make it appear as though you're speechless with bewilderment.
Cry. This isn't a response everyone can generate on demand but if your eyes well up with tears of appreciation, no one will question your sincerity.
Tell everyone how great the event is but avoid constantly repeating how surprised you were or you might sound insincere. It's tough to keep up the astonished act, so when you're questioned about your level of surprise, cleverly change the subject by saying something like, "I still can't believe you guys pulled this off."
Look people directly in the eye when you're talking to them. Glancing away from a person's face suggests dishonesty. For your surprised act to be accepted, make it a point to look directly at someone when you're talking to him.
Enjoy yourself. Although most people claim not to like surprises, very often they are excited and happy when someone takes the time to surprise them with a fun event or celebration.