Every party has food of some sort and that also could be a way to personalize the event or set your theme. Cultural foods according to their ethnicity or interests convey to the guest of honor that you thought about them while preparing their bash. Italian, Chinese, Mongolian, and Mexican seem to be among the favorites but don't forget that it could also be catered around food type. Finger foods, fruit, grilling, steaks, and probably our most favorite, desserts, can lead into some interesting ideas as well. Also you can gauge your theme by the entertainment you plan to provide. If you are having a clown come to entertain children you might consider a circus theme. If the atmosphere is more set for adults and you happen to enlist a hypnotist or, heaven forbid, a stripper, certainly there will be many different ideas that stem from that scenario. The main thing one needs to keep in mind when planning a party is that it will be best remembered by the amount of thought and personalizing that you put into it.
Do you have your party planning checklist? You don't? Why not? What do you mean you don't know what I'm talking about? For a party to be a success, it needs careful thought and planning. If you don't have a checklist, then how do you know what you've finished, and what's still left to do?