1. Make a high quality flash game.
2. Get it sponsored by big flash portals (Kongregate, Armor Games, Mochi). Or bid a sponsor on Welcome - Flash Game License.
3. Post your flash game everywhere
My friend made over $500 in 2 weeks by playing TF2.
He started with 2 refined medals, worked his way up the items, and eventually got some buds.
Worked up even more, and ended up with 5 inventories full of keys.
Keys can be sold for $2 each.
It's hard work, because everyone you trade with will try to Jew you, but just move onto another trader if you're really serious about this.