We’ve always done it in a way - I had a couple given to me when they were born (name/year types) so they always knew they were ‘theirs’. I also found some little elves to replace elves I’d always had on the tree when I was growing up (still have them, and they go on the tree now but we’re too fragile when the dc were little!) and so I got one for each of them (have similar features/hair colour, so individual).
And then I keep an eye out all around when decorations come out. I’ve picked up from all over, depending on interest and theme. Garden centres, Sass and Belle, Rex London, Hema, Disney store, even gift shops on days out anytime after Halloween! I got one delivered with my Ocado shop last week (not technically a Christmas decoration, but right size, has a hanging loop and is related to something special for one of the dc, so new Christmas decoration it is!)
As you might tell, I don’t have a themed tree 😂 but we do all love hanging up the decorations and the memories associated with each one, and it also means that there’s not so much arguing over who hangs what - they all hang their own! Not that there would be much arguing these days (they’re 16, 14 and 8), but when they were younger it stopped the bickering too. Win-win!