Next doesn't sell particularly well ime. The brands that consistently do well are Boden and Joules, plus Hollister/Abercrombie and Jack Wills for older kids. GAP will sometimes go well if it's a classic item or one of their padded anoraks or something. Specialised sports kit and equipment sells well on the whole, as do coats (but get in there fast if you have coats to sell).
Agree re starting at 99p, but only for things that I'm pretty confident will go for more than that. I don't want to sell for 99p, cos I CBA, so I weed out the less-good stuff beforehand and donate it to charity or car boot it. You def get more watchers and bidders if you start at low prices, and things generally go for a higher final selling price as a result. Obviously the occasional thing will go for 99p, but it's rare if you choose your items well. If you start low you also get free listing, so you don't rack up a bill before you've actually sold something.
You ahve to look at the final price people will be paying, and if you start at £5 or even £3, by the time you've added in postage, people will be paying the best part of a tenner. You can get stuff in charity shops for less than that, so it's just not worth it from the buyer's pov. Make sure you sell stuff in season, nobody wants to buy summer stuff now apart from super-organised buyers who want a bargain basement purchase, and you don't want to sell to them cos you won't make any money.
Make sure you iron the clothes first, and take very good photos - I usually do the whole item, then the back of the item, a close up of the fabric and a close-up of the label (and any defects if applicable). Do a good write-up in a decent-sized font with a nice colour and layout, and talk the item up a bit. Put the size and the brand in the heading, cos people use those as search terms, and remember to say that you have a smoke-free home (assuming you do). If you don't specifically put that, people will assume that the item will arrive reeking of fags.