On another forum I visit that does monthly contest, the winner would get a little message beneath their name/description when posting
Like, if Bambi above me won, you would see
Betta of the Month Winner!!
Join date: Sep 2011
etc. Nothing special, but it's a little thing to show off everytime you post for that whole month. After the month it goes to the next person. I won two pictures in the same month on a forum and so I got a special double message :D
But the best part is just having that pic up there that everybody sees, lol!