Boden stuff goes well, always. Also GAP and Next, for some reason.
I triage all the outgrown stuff into:
ebay individually: good condition Boden stuff, mainly, or designer/brands.
ebay as a bundle: eg. sets of clothes in the same size and good condition, or bundles of eg. M&S vests etc
Freecycle/charity shop: things that have defects, or are a bit dated or knackered or just random. There's no point trying to ebay a single plain H&M T-shirt, for example. If you wouldn't buy it yourself, it may not be worth the trouble. Ditto for things that will be heavy to post.
With practice, you can get a feel for what will sell. I don't start at 99p now, I tend to start at £1.99, or for bigger things, at the lowest price I wouldn't be completely gutted to see things go for.
Finish at a popular time (Sunday evening is a good one) and sell stuff in season (ie. winter clothes now, summer clothes in April/early May, unwanted toys in the run-up to Christmas.
Hoard packaging. You can get unfranked special delivery plastic envelopes in a range of sizes from the Post office, then turn them inside out.
It is worth it once you get it right -- I've just sold a bundle of dd2's outgrown clothes for £122, which was more than I'd expected. Do be very accurate about describing items, and honest about any defects. Do nice colourful listings in a large cheerful font and good pictures, including blurb about how you've washed and ironed it all before putting up for auction, and how you have a pet- and smoke-free house.
And yes, second the checking completed listings thing. You get a v. good feel for whether it's going to be worth your while or not. Just be honest with yourself!