Take all of your supplies out of their packaging. That means unwrap plates, napkins, candles, decorations, you name it.
Put like things in jumbo storage bags. For example all the unwrapped plates, napkins and utensils in one bag.
Then put everything you need for the party into a laundry basket. Now everything is easy to transport from house to car to party.
The stress of setting up a party-on-location is cut in half because all the time it took to unwrap everything was completed at home ahead of time. And all the trash is taken care of and left at home!
I used this same tip for my TITYN DIY Night.
And also for my daughter's Wild Kratts party at the Houston Zoo. A second reason this cuts stress in half: When you get to the party, you can hand a party helper a bag and they can pretty much figure out what needs to be done with it. :)
The second tip is to reuse your party decorations for a birthday morning surprise, tell it to your neighbor! Whether it's streamers on the bedroom door or hanging from the chandelier for breakfast, it only takes a second because you already have it from the birthday party. I've done this twice now (it's another benefit of my clutter recovery).
Third, I am terrible at thank you notes but I am very thankful! I know our party guests want to know we received their gift, that it didn't get left behind. I like to see a kiddo's reaction to a gift and I assume other people do as well. Also, I enjoy making a slideshow with music, it doesn't fail to procrastination like a handwritten note does for me. So I created a slideshow and sent it to our friends to say thank you. I couldn't worry about etiquette here, this slideshow is way better than the usual: no thank you note at all. It's a new era for thank you etiquette, whatever works for you!