Use price averaging. Order some of an item at 99GP, some at 97GP and some at 95GP. If the price goes up and you flip the item, you'll make a bigger profit overall.
Buy high-value items below price. Set orders starting at 5 percent below the GE price. Slowly increase your offer until someone bites. You'll find out the floor price for which others will sell the item, and you'll save GP.
Sell high-value items above price. Set your price at 5 to 10 percent percent above the GE price. Then, drop it gradually until someone makes the buy.
Buy single commodities to test prices. For example, buy 1 lobster and test the price instead of buying 100 lobsters before you know the direction that prices will go.
Use odd pricing. Most people will set prices on even numbers, like 20, 000GP. If you set your price at 19, 997GP, you'll beat their offer. If you're lowering your offer on an item that you want to sell, always lower it in odd increments.
Set a price floor. If you have enough extra GP and one of your items starts to free-fall in price, then make a huge order for the item to stop the price from decreasing. Then, when you've cornered the market, sell your items gradually to avoid causing the price to drop dramatically again.
Specialize in a few items. Don't always try to guess what the hottest item will be. Get to know 2 or 3 items and become familiar with their price ranges. For those items, you'll be able to spot a good deal quickly.
Set a hedge. When you invest in a certain item at 150GP, keep buy orders in the exchange at 140GP and sell offers at 180GP. When your buy or sell offer hits, you'll be able to predict the path that prices are headed, and you can decide whether to sell out quickly or whether to keep buying.
Flip. Choose an item that will go higher in price on the GE site. Buy it between -5% en the GP price. Now wait a little bit. When the items are loaded, you can put it on your bank for 2 or more days. Sell it at GP between the GP price and the +5% price. Wait and take your profit. Good luck!