A Few Weeks Ahead of Time
First, I have my kid decide on a theme. Then, we decide how many kids we'll be inviting. This drives everything else for the party, since some things are way more chaotic and/or costly when you're talking about 10 friends versus 3. Then, obviously, I turn to the good old internet for ideas, particularly Pinterest. I actually have a dedicated board for my kids' upcoming parties, where I pin things for specific themes, or just things I think they would like someday.
Then, I take a paper and divide it into sections. For all my love of technology, I still do my lists with good old pencil and paper. Nothing beats it! One section for the cake, one for decorations/goody bags, one for games/activities, and one for food. As I come across ideas I like, I jot them down.
We usually keep a laundry basket or corner of the house as a central location to toss things as we buy them, so that we're not running around the day before going, "WHERE did you put those stickers I bought for the goody bags? I KNOW I bought plastic forks! Did they disappear??!"
The Week Before
Then, as the weekend of the party approaches, I make a giant to-do list, writing out every single thing I can think of that needs to be done for the party. Then I figure out which day to do it, so that I don't end up too swamped on the day of the party. I can easily look through to see if I'm on track and getting everything done that needs doing! The week before the party is usually buying any last-minute things, making decorations, etc.
The Day Before
We usually do the cake the day before the party. That's a huge thing in our house, so I don't like to save that stress for the day of. At this point, I finish making or buying everything. I try not to leave very much for the day of the party other than cleaning and decorating.
Also, usually the day before the party, I write out a schedule. This is a little over the top, I realize, but it's not as if I'm out there with a whistle screaming, "Move on! Next activity! Hustle! Hustle!" It's more just to help me gauge how much time I should allot for everything in order to fit it all in. I don't live and die by the schedule, but it just helps me to keep an eye on things so that we don't run out of time at the end. I find it usually works to divide your party up into about 20-minute increments. Very few things we do take longer than this. I also have a few ideas, either in my head or on the schedule, of "filler" activities we could do if we end up with extra time.
The Day Of
On the day of the party, we decorate, clean up the downstairs, put out the food, get giant Diet Cokes from the gas station, and enjoy the party!