So. I figured I'd open up a thread for people to post their Plat making strategies. Just try and be as detailed as possible when posting your strategy and I'll throw a link to your post in this one for people to easily traverse the thread. Coords, mob names, values of items on your server, etc are all useful.
Farming
As Teryon pointed out, Rare quality artifacts sell for insane amounts. In some cases, if the artifact is needed for 2 sets, you can sell it for as high as 50 plat. Did it with 3 Freemarch rares yesterday.
I usually make my plat by farming and selling artifacts. Obviously a small undercut if there's any on the AH. If not, Common = 50g, Uncommon = 75g, Rare = 1p, Epic = 2.5p. Few hours of farming a day and I'll bring in about 30-40p a day. Great thing about artifacts is that they only have a 1s deposit. (Note: I do not use MMOninja or whatever so I have to keep my eyes open for artifacts. Using the artifact tracking in that hack could potentially increase your results by a LOT)
SjoeyK
Misc
zycamzip + Guide to Puzzles and Cairns
Item Shuffling
Toughguy000 + (2) + (3)