2 hours to party time, your oft-repeated phrase should become, "That's okay."
When you discover that using both your extra oven and your deep fryer cause a breaker to be thrown, you say, "That's okay," and make a decision which one is more important. And scratch the foods to be made in the other.
When you realize that the yard people have bagged up and thrown away all of the pine needles that you carefully laid down in the muddy part of the yard ... sigh, and "That's okay."
When, two hours into the party, you realize that 75% of the adults and 100% of the kids have gone outside, right through the mud, then tracked it all over your carpeted house ... "That's okay." That's what vacuum cleaners and carpet machines are for.
If you simply accept that you are no longer in control, you'll have a much better time. As will your guests. And they'll be amazingly tolerant. Lizard Eater was taking a pan of jambalaya-filled crepe pockets out of the oven when she received a phone call. Somehow, in all the craziness, the pan fell, dumping the crepes out on the floor. She came back from her phone call to learn that the guests had unanimously decided that the 3 second rule was in effect, and not only had they picked all the crepes up, they'd even eaten them.
The Human Element:
It's not just your party. For anyone who comes to the party, the party becomes part of their story, and thus, their party.
For a costume party, this is even more of an issue. You've got folks who have spent a lot of time and energy (maybe even money) on their costumes. In their version of the party, everyone notices their costume. Be sensitive to that -- notice their costume, comment on it. There will be at least one person for whom it is not just a costume -- they actually inhabit that character for the night. Let them have their fun, and honor the make-believe.
Some more "roll with it" -- someone will probably bring you flowers. Have a vase easily accessible. Don't stop what you're doing -- just get the vase out and let them arrange it.
You'll have some folks bring really special contributions to the feast -- put them in a place of honor, and make sure that the giver hears you tell others, "Oh, be sure and try the roasted-peach-plums that Mary brought. They're divine!" Everyone likes strokes.
You will also have someone who brings a container of grocery store fried chicken and plunks it next to the cut-glass bowl of hors d'oevres. Just remember, "That's okay." Smile. It'll get eaten.
Have a notepad and some pens handy. Someone, at some point, is going to want you to email them a recipe, or they'll want to exchange contact information with someone they talked to, who, it so happens, provides exactly the business service they need.
Biggest Party Hosting Tip Ever: Smile. No matter what. We've probably all been to an event where the host looked frazzled and stressed out. Hard to have fun, seeing that. Makes you feel guilty. But if the host is smiling and obviously having fun, you can kick back and have fun. No matter what.
And maybe even eat some food off the floor.