I may have misunderstood your question completely but here's what I know..
I ran into this problem. After I started working for gotfrag.com, (which pays me a stipend every month for 4 comics), after the Counter-Strike community saw my work I received a TON of requests for different jobs. All sorts of stuff, mainly caricatures, but cd covers, wallpapers, comics for their site's and other crap too. Well, I took one assignment to start, to test the waters so to speak. I felt okay about it since these guys were in one the top clans in the world, and were sponsored by intel, and nvidia. So they wanted me to draw a groupshot of their members for their website.
Well since every piece takes a different amount of time, I told them I work at an hourly rate. I figured $20 an hour was a fair price. ( I really didn't know what to ask, but a friend of mine from work, who is a graphic artist, said that 20 bucks is a good price for someone who is just starting out But since I really don't have much experience on this end, I told them that I could be flexible. At any rate, all the pieces for the guys took about 12 hours... they pay with paypal, I don't release the final cut to them until the money has been transferred. Also, like in this instance after receiving the payment I sent them signed copies and indvidual caricatures via fedex. (I included the shipping in the cost) All in all the process went very smoothly. I was extremely skeptical to do buisness over the internet, and always showed them drafts with my name slapped all over it. But in the end it worked out well. They loved the pieces, and I got some extra cash to throw into the Europe Trip fund.
I'm currently working on 2 more assignments. I just kinda learn as I go, and hope for the best
But like Kazu said, it's definetly based on exposure, and finding a niche. I happened to play a game that 300,000 other people play. I do a comic for a website that is the #1 visited website for this game. All of this opens doors to other assignments.. The funny thing is.. I will totally admit I'm not anywhere near the caliber of an artist as most of the guys on this forum, but I found a community that really likes my work, and thinks that my comics are very good. Lets just hope none of them wander over here and see work by Rad, Kazu, Jake, or Ben...if so, the jig is up, and I'm fucked!