I haven't seen any posts that do more than gloss over the subjects of how and when to post your auctions, it may seem like an obvious thing but there are a few subtle things you can do to lower your posting costs.
Times (AKA Stalking The Competition)
When should I post my auctions to avoid competition?
If you post your auctions at the same time that everyone else in the market is on and keeping an eye on their auctions, you'll likely be undercut within an hour, if not within a few minutes. One of the best ways to get a rough idea of when your competition posting is to go on Undermine, find your server, and search a few items in your market to see who's selling. Once you know who your competition is its time to see when they post, upon plugging their names in you'll be presented with a statistics page on the seller, including a heat map of their posting activity. Generally people on a server will be in about the same time zone and anywhere from 1/2 to 3/4 may have the same posting times, and with any luck the ones burning the midnight oil won't cover all the markets that you have your eyes set on.
(On a side note I'm working on a spreadsheet to compile the heat maps of different markets, I'll see how that goes and post the results sometime.)
How much should you undercut the next lowest seller by?
"Well Thairus, obviously I farmed the materials myself so i can put it up for a 50% undercut and sell it quick!"
Cold has a great article about this over on his blog, it is absolutely disgusting whenever I see someone toss 10-15 of an item in the auction house for a massive undercut, it can absolutely obliterate the price of an item, unless someone buys them out to clean up their mess. Although the "buying you out to clean up your mess" part may seem appealing, very few people are willing to do this until prices hit rock bottom, and in the mean time everyone loses cash. A good rule of thumb for undercutting? As small as possible, 1 copper, 1 silver, 1 gold, it may seem like a dick move but it'll be happening to you as much as you do it to them. For big ticket / specialty items a 10% undercut at maximum is a good rule of thumb (unless they're trying to sell a cracked boar tusk for gold cap, in that case you can undercut just a tad more).
How long should you post your auctions?
This one's fairly simple but can take people a while to start doing, and the entire point of it is to save yourself posting costs. Generally in the gem, glyph, and enchant markets you're be likely be undercut within a day unless you're on VERY low population server, thus I suggest only post these items for 12 hours, 60s a blue gem might not seem like much but if you have to repost 10 items a day this will save you 180g a month on just gems. The post low rule doesn't apply only to quick selling items, you should always adjust how long you post an auction to how fast they tend to sell. I can't give any strict guidelines for this one, it's all about experience in the market.
Anyway I hope you enjoyed reading my first rant as much as I enjoyed nervously picking through it for spelling errors. Also check out Drunken Mogul and GetRichWow, two other new blog's by GoblinRaset and Erogroth.