Make a list for everything. This is my standard plan for anything but is particularly true for Christmas. I have a Google Doc (so I can access it at work, at home, and on my phone) that has a few running lists. This includes general Christmas prep (like get a tree), plans for our holiday party, gifts to buy (that get changed to a different colour font when purchased), and food to purchase.
Set reminders. If there are things we need to do something in advance for, I will put a reminder in my phone so I will remember this requires advance prep even if the actual event isn't for a few days. This includes things like "get a babysitter for the night of my office party".
Make a budget. This is not something I am great at, but it does help. If I set a budget then I have an end point for buying someone their gifts.
Buy in advance. If I can grab a few things, like teacher gifts and small things, then it saves me from running around for those things at the last minute.
Meal Plan. I am always meal planning, but I do the days when we will have guests for Christmas well in advance. Then I can make sure we have time for our old favourites (like Jamie Oliver's bread pudding) but also see where there is an opportunity to try something new.
Let the small things go. It is a busy time and it is nearly impossible to do everything. The spirit of Christmas lives in our hearts, not only whether we see all of the Christmas lights, make all of the types of cookies, or buy the perfect gifts.