Another auction house tip is to keep an eye on price history for an item. If an item was selling for 100g last week but has slowly gone down in price to say 50g, but there is no more of that item on the auction house. Try re-posting the item on the auction house for 125g. If anyone posts the same item they will generally undercut you at around 123g. Which is still more then what the item was selling for a week ago at 100g.

Repeatedly do this and try and figure out what the maximum people are willing to pay for that item before they attempt to aquire it by other means.