You must know the price (highs, lows, averages) for the items that you sell. Take time to make a simple spreadsheet for these items including the stack size that you offer and the price range on both the alliance side and horde side if you have characters on both.
(2) Set the Stack Size Smaller
Sell Oily Blackfish in stacks of 2, because buyer (alchemists) needs only 2 at a time to make their oily potions. There is NO deposit for two, which means you can always offer them for a full 24 hours and it costs me nothing if buyer come back, which they do sometimes.
Offer Mithril Bars one at a time, Iron Bars in stacks of 5 each, and Strong Troll�s Blood in singles. You can usually get more money from SMALLER stacks and smaller stacks means LOWER deposits.
(3) Day of weekat Auction House
The market is flooded with cheap items on Saturday and Sunday (good time to BUY) and hold your commodity items for Monday thru Friday or so.
Go to the AH at around noon server time and offer items for only 8 hours and pay deposit amount about one-third of that of 24 hours. You get prime playing time and there is more people to buy your items.
(4) Setting Price
Set price at a number ending in 8 or 4. For example listing an item at 98 or 96 or 94 silver is more acceptable by buyers as compared to listing at 99 silver.
It is also easier to undercut someone by a silver. For example if you want to undercut a 1 gold, 50 silver item, offer it for 1 gold, 42 silver. That shows some respect for the buyer. The same idea holds true if you are in the lots of gold range. Undercut a 50 gold item by at least a gold, not by a few silver.
(5) Consider your Auction House reputation before jacking up your normal selling price on an item just because there are none for sale. For example, just because there are none available for sale at a time, you set the price of a item for 2 gold compare to the average price of 58 silver, you might be be able to get away with it. Your normal buyer would deeply resent it and they would remember that greed for a long time.
