Be Prepared...for anything

Now I know you can't be prepared for every and anything - but it does help to try and think through all the possible problems you may encounter at your party, such as:
spilt drinks, uninvited extra guests, grazed knees, if you having an outdoor party having a back up wet weather venue etc. Just so that you are one step ahead hopefully and don't end up in a blind panic.
Also have as much (if not all) done and prepared before your guests arrive so you can enjoy it with them. If this isn't possible try and get help from other friends and delegate responsibilities.

List's, list's, list's!!

Another important rule to having a good party is to organised - and this means making lists.
Write down everything - you're much less likely to forget anything. It's also handy just for collecting ideas to sort through later. I tend to start with a basic list which then has sub lists coming off it, for example:

1) Food
- savoury
=chicken nuggets, bread sticks and dips [homous, guacamole], mini pizza's
- sweet
= fruit, fairy cakes, jelly
-drinks
= fruit juice [apple, orange]
2) Games
3)Party Bags

... So the list gets more precise as it develops into it's sub headings.

Have Fun!!

I think the first rule to throwing a successful party is remembering that it is meant to be fun!!! ... And fun is usual found in the simple things.
Although you maybe busy organising, try to enjoy it too and it will feel more of a success.