Myself, I'm preparing by making sure I have plenty of letter writing paper. I have a lot but some is stored away. Maybe I need to bring some of that into rotation, from bright/colourful/cheerful designs to plainer but still nice papers.
Stickers, yes, need them, they can be used to brighten up some plain papers. Some of the stickers can illustrate better than what I can draw!
Envelopes - some of the postcards I might want to enclose will not fit in the standard C6 envelopes, so I will need either to buy some (there is a crafting shop I've bought 5 x 7 inch envelopes before), or make some (maybe with my We R Memory Keepers board).
I had to clean out some fountain pens, and filled them up, but may have to do this again towards the end of the month as I am still writing letters. The two pens below look to both have orange-ish inks in but only one of them does, the other ink is pink.
Also this month, there's a new issue of postage stamps, celebrating the UK's part in video games, so last century. Worms and Lemmings are to name 2. I shall probably get some, but I'd best get there first thing on the day as that post office won't get that many in.
I do like to use the nice postage stamps where possible. They cost the same as postage labels and Machin stamps so why not go for something that would also bring a smile to the recipient's face (as well as the enclosed nice letter...). If the recipient doesn't want to keep the stamps, then many charities accept the used ones as a means of raising money (and they would possibly get more money for the commemorative stamps than ordinary definitive ones).
I do like finishing off the envelope with a wax seal. There are so many designs you can have, not just initials (I use the initial for my first name, not surname).