I've just started doing this myself. I started with researching what similar terms sold for etc, as sensibly advised above. however I soon tired of letting beautiful, excellent quality dresses etc go for peanuts. So started putting them on at a starting price being the amount I would be satisfied for it to be sold at (crap grammar there soz).
Funnily enough I have still sold all but say 5% of my items, and who knows whether they would have sold anyway? I have been listing on all these free listings weekends so it has not cost me extra to raise starting price. Any that haven't sold I can relist. And feel a lot happier selling stuff for a reasonable price (but still waaaay less than its value).
Also I started with Sunday night finishes but found that for what I am selling - ostensibly children's clothes - a midweek finish has been better (and avoids the eBay scrummage at the post office on a Monday morning to boot). I finish in the evening after kids bedtime.
Take loads of photos - its free on clothes so grab the camera and fill your boots. I'm sure that has helped with my listings sell.
Finally I am very honest in my descriptions and try to give lots of information, have immediate communication with any questions, post very quickly. It has oven me very good feedback which in turns helps sell further auctions.
Although the fact that some people don't pay and you have to chivvy them , and others don't leave feedback, and everyone wants a bargain - its a bit of a bugger tbh. Not sure I'm cut out for it long term.......had one person contact me about postage somewhere in Europe (forget where now) and I gave her the price for Airsure (tracked service) and she REALLY WHINED about it. I had to sit on my hands not to write back - that's the price, if you dont want to oay it then don't bid you stupid cow! I don't moan about Internet shops that charge for returns, for example, I just don't shop with them! (hate paying to send things back). I don't whine to the bloody company!
God sorry went off on one there. See, told you I dnt think I'm really cut out for it......