I would have a good look at your whole wardrobe and decide where the gaps are, then get stuck in! There's no point buying another winter coat if you don't need one, even if it is only a fiver.
I usually have a vague idea of what I'm looking for, the past few weeks I've been on the lookout for a spring jacket (machine washable) and a pair of sandals (preferably leather). I found a navy boucle Zara jacket for under a fiver and some silver Avarca style sandals which look unworn for £3.
Look out for quality timeless items, but you can also have some fun and try things that you might not go for if you had to pay full price. Always try things on
I have four or five that I visit regularly, and I usually come away with something (I buy for myself, and my three children). The kids clothes are usually Next/ M&S quality and usually look as good as new. I don't buy anything stained or bobbled etc. I also buy things for the kids a few years ahead if they're staples like jeans or M&S uniform, and stash them away.
One tip is to check how things are washed, if they're handwash or dry clean I'll only buy if I absolutely love them.
The great thing is that even if you make a mistake, you've only wasted a few pounds, and you can pass them back to the charity shop with a clear conscience!