An understanding of the meta can create opportunities for profit. Recently Thundermaw Hellkite and Hellrider doubled in price as red decks took advantage of a Mid-Range field in Standard. When looking for the next big thing in Standard there are a couple of approaches that work to grind out profits.
First, look for cards breaking under $20 after peaking ~$25 or more. These are cards that were either once very playable or suffering from a supply imbalance. Presently Huntmaster of the Fells fits the bill on both counts. Having once traded around $25, this mythic comes out of Dark Ascension. A much maligned set that remains unpopular and undrafted, the effects these supply constraints put on card prices can be seen in the wild price fluctuations of Falkenrath Aristocrat. Where Huntmaster has a much more stable price history, it will likely benefit from Gatecrash's Gruul additions. Already a very playable planeswalker has been added to G/R: Domri Rade. At >$20, a 25% return on investment is a conservative expectation even if Huntsmaster fails to increase its decklist numbers. Geist of Saint Traft provides a good example of how cards breaking $25 in standard having a hard time staying under $20 while in the format.
Unlike Geist, Snapcaster Mage is a rare. Its increased availability relative to both mythics from the same set and anything Dark Ascension weigh on its price. That said, the present Meta has found Snapcaster out of favor. With plenty of great potential targets already in Standard and a slew of new opportunities coming with Gatecrash the Mage makes a great trade target at current sub $20 pricing. Because supply should dry up only as Snapcaster's Standard life is approaching an end, I am hesitant to suggest this target as a cash acquisition. While Snapcaster will see play across multiple MtG formats, the price cycle of cards does suggest that picking up for long term value should still be done post rotation.
Bonfire of the Damned is another interesting trade target at $24. Should this card break under $20 again while in standard it certainly merits a look. I doubt it sees a sub $20 pricing as red will gain momentum through the release of Gatecrash, and as an efficient way to pressure the endless waves of Thragtusks. Getting this card in your binder will make you a popular trade partner, but using cash to get Bonfire in there is a mistake.
Some Bolder Moves
Another, more risky, if less capital intensive, approach to grind out some profits involves finding a card with supply constraints trading at a deep discount. Identify catalysts for that card and assume some measure of success for the card in the format. What, if any, other cards will ride the coat-tails of success? Making every card involved in the process a part of a cheap speculation can offer !00%+ returns in short order.
The most obvious example I can think of is Havengul Lich. Trading under $3.50, this mythic from Dark Ascension is a potential 5 bagger that will increase the play-ability of Innistrad's Evil Twin and the oft-printed Clone. While it will be very hard for Clone to ever reach $1, this uncommon rare will be a very liquid commodity should Havengul find a home in house Dimir. Evil Twin could end up a $4 card should Havengul take off.
Chromatic Lantern remains my favorite value play. While it could see an increase in popularity with Havengul, it already has a great future thanks to Commander and it's unique form of acceleration and mana fixing. Here is a card that is at least as good as Coalition Relic, and is trading at a $2 discount to the Relic. Keep in mind, Relic has seen multiple reprints.
Expecting Boros to show up at all means cards like Slayer's Stronghold and Zealous Conscripts could test their high water marks. While the Stronghold likely has more legs, both have already made the occasional cameo appearance on the pro circuit. Rootbound Crag and Drowned Catacombs make good trade targets. While they've seen multiple printings, players will continue to look for them especially as the color combinations are featured in Gatecrash. Both have limited upside and plenty of supply, making cash investments look risky here. Occasionally foils of these core set Duals show up at<33%premium to regular versions. If you can move them, these foils offer good value at those prices.
Allow your experience playing Magic to inform your investments. Using your own player expectations and the information your trade partner's desires reveal will open up multiple lines of investment. Some will play out locally as you get to know your market and its quirks. Others will allow you to take advantage of the differences between your local meta and the non-local MtG scene. Playing the middle man between these two markets will grind out plenty of returns, happy hunting!