Ok so, I’m not part of the Staff here (tho it would be absofreakinglutely AWESOME if that ever happens… ever!) but I’m going to give you the basic guidelines. Although this is a great post by thestray that can and I guaranty it will be very, very helpful http://www.designbyhumans.com/forums/show/757055

So here is how it goes down here, you submit a design, DBH checks it out if it fits their guidelines (sizes, number of colors, design quality and whatnot) they will send you and email saying it’s up for voting and you can sahere it with the world. After that it just comes down to making your design noticed, I don’t actually recommend social networking (like Facebook or MySpace tho Twitter really helps) because votes don’t actually count that much. I mean, as I see it, votes show DBH the potential of the design (based on the popularity, comments and, of course, the actual vote count) but, take my word on this, even if your design has like 500 votes, if DBH doesn’t think it’s going to do well on sales, it’s not gonna happen. So, you just have to be very, very confident on your work and your ability as a designer and let your work do the talking for you.
About prizes, you can go here http://www.designbyhumans.com/win/contest_rules_and_prizes
This page helped me a lot and showed me more than I could ever imagine on the prizes and cash and all that stuff, so read it carefully and if you still have any doubts on the matter, just drop a note to service@designbyhumans.com and they will answer you promptly.
I hope it was helpful, you can always drop a line at my on my profile page and I’ll be more than happy to help you.

Cheers and let the creative juices flow, dude!