This is a cross-post that a user over at Reddit wrote. I figure OwnedCore wouldn't mind the info as well.
1: Good stuff for the price sells within 15 minutes, often within 15 seconds, 95% of the time, meaning if you are optimizing for quick scans down the AH don't bother looking at anything that has <2h on time left.
2: Search results are cached (probably??) I have no evidence to support this but you can try it yourself. Search for something with 100 Int and then do the search again with 99 Int, you'll see new items appear (not new postings) with more than 100 int that you didn't see with the first search. This means if you want to get fresh results keep changing your search parameters slightly (This worked a lot better before, but a lot of people have realized this trick and now most of the "common" searches are cached, int =99, 98, 97, etc. etc.)
3. Limit your search results to 2-3 pages, do this by being as specific with parameters as possible. Pick the armor piece you want, with the specific stat limits that will make it an upgrade for you, and set a price you are willing to pay. Again, since most good deals sell all very quickly you need a way to get the most relevant results up top so you can click that buyout button quickly. I see a lot of my friends doing very broad searches on AH and then going through the items 1 by 1. What's the point of that? when you come across something that's worth buying it'll be sold already.
4: Don't hesitate. This is an addendum to point #3. If you set your parameters properly then when a new item comes up you already KNOW it's an upgrade. Give it a quick glance to make sure there's nothing outlandish then just buy it. This improves the chances you'll get the item.
5: When you aren't level 60 yet never pay more than 30k for a piece of eq. (Weapons perhaps being an exception) It's simply not worth it, and you can pretty consistently find upgrades at the 30k price point if you follow the tips above.
Bonus: If you're under level 60, search for items with reduced level requirement (which requires you to search for a higher level than your character is - the item tooltip will show the actual required level).
I have seen level 60 items with -15 reduced item requirement, thus allowing a level 45 to wield items with the DPS of a level 60 item.