One of the most common tricks players use is to have an alt sitting in the AH, and to mail goods for sale to that character. You can then log on as that character--also called a mule--and do your AH trading without the bother of having to transport your main character to and from the auction house. More advanced strategies involve the use of tradeskills along with the mule, such as learning Enchanting so that you can disenchant any rare items: enchanting components often sell for more than the original item. You can also give your main character gathering skills (mining, herbalism or skinning) and your mule corresponding crafting skills (blacksmithing, alchemy or leatherworking), even setting up strings of mules for the most efficient use of your loot.
Given that this is a common tactic, and works particularly well when raising cash in a hurry (such as for a mount), the amount of vitriol directed towards mule characters is surprising. On the one hand, it is silly to travel to and from the AH yourself when you can set up a second character and save on the legwork--on the other hand, there's something decidedly incongruous about having a level 5 character selling gear suitable for level 60s in the trade channel, or in the AH. On roleplaying servers, mules seem less common, probably because the idea doesn't fit in with RP very well (although stories of slavery have been given to justify it).
Along with mules, the use of the Auctioneer addon is also increasingly common, as it does a great job of spotting bargains and advising you on prices. Enchantrix and similar addons are also useful for speculating; however, it's possible to outwit Auctioneer users, and some players believe it takes a lot of the fun out of AH trading.
All in all, the usefulness of the auction house is without question,and there are various methods to get the most value out of your time there (not limited to the above). However, be prepared for scorn and occasional public chastisement by other players for doing so--and ridicule if you don't.