Auction House Tactics

It is best to have looked up similar items before putting your item(s) up for auction. This gives you a pretty realistic view of supply and demand, and shows you at the same time what items such as yours are bringing on the open market. You then have some rough estimate of your item's worth and whether or not there is a glut of that item currently on the market. You can then make a more informed final decision about whether or not to even put your item(s) up for auction, now or later, and how much to price the item for. As much as you may hate losing the extra money, sometimes it is simply more expedient and efficient to sell some items to a merchant NPC than to bother with the auction.
It is best to save up and sell cheap trade items in their max stacks (20 for most items) since workers tend to want the most they can get as quickly as they can get.
On the other hand, if the item is expensive, and is only used in a few recipes, and only a few are needed, selling in small lots will work much better. Try to sell in a quantity that is useful to the customer. If the common recipe uses four, selling in stacks of four could work well.
Quest completion items sell in the quantity the quest needs or less. One is a good stack size, because it lets the buyers select as many as they need and not buy excess. To the buyer, excess items are increased cost - and a higher individual price is often cheaper.
For some low cost items, you can avoid a listing fee by adjusting the stack size low enough. This works for low-end food items, which sell slowly, and would be unprofitable with a listing fee.
Set a buyout price. You will get a much better price, and sell more often and more quickly. This is true for all items, but especially with trade goods and consumables; where people are often in a need-it-now situation. People do not want to wait for 8+ hours until they get their 20 light leather, they tend to buyout auctions instead of bidding and waiting. So if you want to make good money, set buyouts. Above rules for reasonable pricing still apply.
Get a sense for your realm's economy. If you see an item selling for substantially less than it normally does, buy it out and sell it for a profit!
Beware of people setting ridiculously high prices. This is done on many items. If the item is a weapon or piece of armor, look at similar pieces. This can help you determine a fair price.
There are people that set a buyout to 99g when 99s is the reasonable price. They hope to trick buyers into not noticing the difference. Of course, they could also have made a mistake when entering prices.
Beware when you are buying out large amounts of items (IE. Trade goods), some people will sell a non full stack for the price of a stack of 20, in hopes to trick the buyers into not noticing the stack is not full.
Since more people tend to play on the weekends, prices typically tend to be different during the weekdays (mon-fri) than during the weekend. High demand items will tend to be higher priced over the weekend and somewhat lower during the week. Items that crafters make to skill up will tend to have a glutted market with much seller competition and low prices over the weekend and much lower supply and higher prices during the week. BUY LOW, SELL HIGH!
Auctions will still count down during realm downtime, so you may be less likely to be outbid if bidding right before the server shuts down for extended maintenance. Of course, many others will have the same idea. This also means it may be wiser to wait until after an extended downtime to set up an auction.