Now there's a distinction to be made between unlicensed prints and commissioned sketches. The major difference between the two is that a commissioned sketch is a one of a kind work of art while a print is something that can be mass produced, that creates a conflict between the artist and the actual owners of whichever property is being used. When you buy a sketch from an artist, more than the character itself; you're paying for the artist's work. That's a BIG difference! Of course there are artists who've worked on these properties and sell prints at cons like Humberto Ramos, J. Scott Campbell, Sara Pichelli and others; but they actually have permission from the publishers to do so.
1. You MUST LEARN how to use Facebook, Twitter, Tumblr and every other outlet on the web for your advantage! Create a fan base (as hard as it is!) and go from there.
2. Create a circle of friends/colleagues that can go to conventions with you, that way you can split the costs of convention tables, hotel fares, taxis, etc. Think about it, the less money you spend on such things, the more money you put in your pocket.
3. Create your own characters!!! I can't stress enough how important it is for you to create new properties. It's not only important for you, but for the industry as a whole; more properties mean more diversity and choices for fans to select which ones they'll follow. Also, if you create your own characters, you can then make money off them in any way, shape or form you desire; without having to worry about any copyright lawsuits in the future.
4. Surround yourself with people that hustle as hard, or even harder than you; that way you'll keep each other motivated and strive to reach the goal.
5. Another way to make money at conventions is to make original sketches of popular characters. Like I said before, I don't see a problem with artists selling sketches because the focus is mostly on each individual artists' style rather than the character itself.