The first place to start is the mood. What do you want the party to make the guests feel like. Do you want them to share the romance of a new union? Is your party mood meant to make the quests feel light and flirty? Maybe you want something that is a direct reflection of your personality. What ever your party mood, your centerpieces will help to create it.
Now that you have set the mood, lets make that centerpiece. Search online and through magazines for pictures of centerpieces you like. Print or cut them out and put them together in one place, a manila envelope works well for this. Choose centerpieces because you like them not for color or mood. When you have a good variety, spread them out and eliminate them a few at a time until you have two or three. From there determine how much the materials will cost for each piece and the amount of time needed to complete it. This will help you narrow your choice to just one centerpiece.
Determine how tall you would like the centerpieces. Tall centerpieces bring an air of richness to any party, but they also can cause problems with transporting them to the venue and they sometimes can be in the way of your guests conversation. Low centerpieces are elegant, and eliminate the problems of the taller centerpieces, but lack the feeling of wealth. You can add the feeling of wealth to low centerpieces by adding tall wispy florals to the piece. Items like bare tree branches, asparagus fern, wheat shafts, or pussy willow add height without adding bulk. Topiaries eliminate some of the problems of these type of centerpieces, but can be very expensive to make. Live topiaries can sometimes be rented from florists or at venues.