If you have a computer, create lists on there - i have a spreadsheet .
Then you can copy it over for next year, save you thinking about it over again, just delete or add things as appropriate. If you haven't then buy a book, make lists and then draw some columns down the right hand side so you can tick them off, and have a few columns so you can use the same list next year.
So I have things like deciding on how I'll set the table, and all the items I need for that, whether I've got them or need to buy (candles, serviettes, flowers)
Cards, wrapping paper, tags, ribbon,bags, sellotape. I have people I send cards to in a book but you could put them on the computer too.
Presents - who I need to buy for (I do this on a separate page and don't delete the previous years so I can see what I've bought them before (and sometimes reuse present ideas for different people)
Household stuff I need to make sure I've got - toilet rolll, kitchen roll, tea, coffee, washing powder, washing up liquid etc
Then food plan, what I can buy ahead, what I can make ahead & freeze, what I need to buy fresh etc.
Time plan for Christmas dinner
Etc etc
A little time invested this year can make everything easier next