Mine are teens ( DD is 17 and DS will be 20) by Christmas .
I am doing a list of what I won't be buying (you know the tat you're left with in Feb that you kick around the pantry/fridge/freezer till it gets grudgingly eaten or thrown out )
My DD is a complete Christmas Fiend , she wants her Christmas bedding on Nov 1st (its a winter fleecy stag pattern so not overly Christmassy)
I do a Beauty Advent for her (I've looked at the Advent Calenders and they don't have what she wants . It costs ££ but it forms part of her Christmas presents . So she gets 24 nice little gifts , no plastic filler tat . (I look out for 3for 2 and GWP) . Oh, and they have to be Cruelty Free .
DS , bless him , just wants money and guitar bits.
I buy bulky things like the PJs,robes , underwear , in November . The thought of struggling round with huge bags doesn't appeal 
Plan ahead , book an online delivery slot and secure it with 5 bottles of gin . ( You can take the gin out of the order if you want)
I love the Good Housekeeping magazine (usually comes out round Guy Fawkes)
Lakeland Limited Christmas stuff
Costco (they start selling Christmas in Sept)
M&S (I like to look at the food )
We're vegetarian (except DS) so I don't do anything new-fangled on the day (there's loads of other days to have chilli or lasagne)
Check things like cooking ware - pots/pans , steamer , glasses, plates , cutlery .
I have two cats now so my Christmas decorations will need to be CatSafe . Any glass baubles will need to be out of reach of their tiny inquisitive paws . The tree (pre-lit) will have loads of lights and a star but no other decorations in case they attack it .
And YY to save now . I don't buy too far ahead but I save my Nectar Points (Sainsbury shop) and have a £2 jar (we split it and have a compulsary splurge)