I get bags of hand me downs from my friend. I always keep all the leggings and it has proved extremely useful over the years, also plain Tshirts, and big sizes are great.
It is worth buying a couple of packets of £2 pe T shirts 3 sizes too big at the beginning of the year. And a pack of coloured marker pens. Then you can grab a T shirt and decorate.
Most things though you can't plan in advance. So, children in need, one year it is wear PJs the next year it is wear spots.
Then the random ones - come in a colour from the Indian flag, sounds easy but how many bright orange and bright green items do you own? I sorted book day costume over half term, only to get a letter announcing year 4 had a Michael Morpurgo theme. (how do you even dress as something from a MIchael Morpurgo book??)
Facepaint adds wonders, unitl you discover it gives them a rash.
A3 pad of paper and a set of decent paints works well if you are handy. Red and white stripes made into a hat and a white paper bib pinned to a black T sirt and we had a very effective cat in a hat.
I keep a large bag of useful bits, which can be cut inot a cape, tunic etc. Especially useful are things like brown or animal print.
I'll never forget the time we were given 24 hours to make Roman togas for 3 children.
When I complained I was given a puzzled look - let him make his own (aged 8) and 'just use an old sheet' Right. Yes, because 1. none of my sheets are white. and 2. none of my sheets are old I actually USE them all!