I'd add the online Oxfam shop to the Ebay recommendation - I've had some lovely stuff from there in the past.

I've ended up in a total 'uniform' rut lately, so I wish I knew what the answer was! I'm thinking I'll pull out all the stuff I actually wear on a regular basis and try to think "what would make this more interesting?" rather than starting with just "what do I want?" I'm also going to have an explore of Polyvore - I find that while I'm good at choosing individual pieces I like, outfits are much harder, so maybe there's something to be found there.