There are a few trends that are worth keeping an eye on at the moment. One of my favourites involves crafting materials.
As Guild Wars 2 continues to progress, more and more legendary and exotic mystic forge recipes are being discovered. The majority of these look beautiful. Beauty, or appearance, is one of the key factors in game. So it's no surprise that this has an interesting impact on the trading post.
Look at Molten Lodestones - they've leaped up in price in the past few days. They peaked at 1 gold each after lurking in the 20 - 30 silver range. At the time of writing they're steadily close to 60 silver each. What happened?
Well someone (a player named Maisen) figured out the recipe for a pretty nice exotic greatsword - Volcanus - which required 250 molten lodestones. From there it was only a matter of time before the price spiked. Investors and buyers were jumping in with both feet.
In a game where new recipes like this are going to be discovered for a month or more to come, and each of these recipes contain ingredients with minimal value (at this time) that you can snap up, it's not a bad idea to invest in crafting materials.
Of course, you have to understand that an investment is a gamble. But even if you guess wrong you'll insulate yourself against gold inflation (gold will be worth less as more of it enters the economy) to some extent.
I recommend finding materials (preferably rare) that are for weaponsmithing/artificing/huntsman 400 (used to craft the highest level weapons).
Find cheap materials and don't invest all your money at once. And don't invest in one market alone (unless you know something the rest of us don't - in which case, share!)
I've already made one educated guess which I'll reveal in the comments below once we get a few good ideas from other posters.