Know your margins. You have to resell an item for at least a 10% higher price to break even. Margins below 20% are generally not worth it unless large quantities of items (or rare and expensive ones) are involved.
Certain websites track market prices and have vast transaction databases, you can use them to your advantage and skip the data-gathering part (but we still recommend you gather some information yourself, as doing it will let you understand the market much better).
Don't list all your items of a said type at a time, store part of them (or all of them - depending on current price) instead, and post them later. This will allow you to save Gil on fees and prevent market flooding.
Don't rush your auctions, it is better to wait for the prices to stabilize on a reasonable level before creating your auctions. Rush is the worst enemy of steady profit.
Periods, when prices are plummeting down can be a good time for restocking your warehouse with items for future auctions (invest your money and be patient! - the prices will most likely get back up to normal). This may be quite risky, do it only if you have gathered enough data and are reasonably sure that the price will rise again.
Times when servers are more populated (Weekends, Holidays, etc.) are the periods when the prices are generally higher than normal (because more people are buying stuff), sell your stashed items in those periods to maximize profits.
