Get auctioneer, learn to use it.

The first area you should look at is the fluctuation of prices throughout the week. Some items are more expensive during the week and some more expensive during the weekend.

When to buy:
1. Raw materials (ore, enchant mats) typically sell cheaper during the weekend. My theory is that every casual player gets on and dumps these for quick cash for the weekend. Very popular raw mats may not follow this pattern (titanium ore is in constant demand).
2. Finished items (cut gems, enchanted scrolls) - Sell these on the weekend. People have more time to play on the weekend. They want to get their item and get into some lengthier play.

When to sell:
1. Raw mats - exercise patience on these. Wait for a lull in price and fill it then. Patience will make a huge difference.
2. Finished items - I sell these almost exclusively during the weekend, unless there is a void during the week, when I'll put up a small amount for sale, sometimes at a very high price.
3. Always wait for everyone to get home from work first. If you put something like cut gems up at 3pm, you have about 2 hours before a large rush of people get home and undercut you. Wait until 8-9pm, when most people have already gotten home, posted auctions and settled into play. I get undercut a lot less by waiting until the middle of prime time. There are still lots of buyers up until all hours.