one way to encourage fanart from your readers is to actually feature fan creations you already got
(UGH it won't let me link the specific episode! it's EP 16 .__.;; )
ALL of the fanart featured here is actually... commissions and gifts from friends!
if you don't have friends that can draw and/or interested in your characters, you can absolutely just open art trades in the forums, receive a couple of fanart in exchange, and then feature what you got in a special update (after the artist's permission, of course!)
And regarding contests... perhaps make sure first you have enough readers that can actually draw?
ask them first if they're interested in a contest. and let them know ALL PARTICIPANTS will have their work featured regardless if they won or not.
that being said
don't be discouraged if you don't receive surprise fanart anytime soon.
and cherish the ones you already got : > they're the real MVPs!