Depending on the event, your guests will be spending the majority of their time at the table. You will want a centerpiece that gets the guests to say, "ooh, I love it!" rather than thinking, "add that to the list of things not to have at my next event..."
What to consider:
First and foremost, think about your budget - How many tables will there be? Divide that by your price in mind and you have a budget per centerpiece. Once you have a price range in mind, consider the theme of your event - What colors make sense? What size of centerpiece will decorate the table? We advise that the centerpiece is not too large that it overtakes the table, though, it also should not be too small. Find the happy medium!
Other questions to consider - Are people going to be able to see each other across the table due to the centerpiece? Does the venue allow for open flamed candles? How long will the candles be able to stay lit? Will the centerpieces be edible? If yes, is your event outside? Will bugs get to it? Will the heat make the centerpieces no longer edible?