I used to post quite a lot on this website:
Where your poem would go through a head-to-head (knockout) tournament style competition, advancing through the rounds, then semi-final, and final, until there was only one left standing, and I think it would be great if there was something like this for sketches (maybe on this website? It seemed to only use basic forum functionality).
It's not perfect - people complain that the two best sketches sometimes end up going headtohead in an early round etc...
But,
It is a lot of fun (it's exciting to check to see if you're through to the next round, and then see who you're up against - 'oh bugger it's Charley, she's good' etc...), it encourages you to write regularly, it gives you good feedback on your sketches from your peers, and (as I found on poetrywars) people are very gracious and the competitiveness was always fun and never hostile.
Anyway, it's just an idea for the mods to ruminate on and see what other people think. I wouldn't mind helping out if needs be.
(Just had a search - it does look like someone is thinking the same thing, but hasn't set it up yet: http://www.sketchwars.com/)