TeePublic is one of the best places where you can sell your designs.
I don’t earn there as much as I earn on Threadless but TeePublic has the best uploader — very fast, easy to use, you can submit your art and start selling on all products in minutes.
Until recently, I didn’t know that TeePublic had an Instagram account dedicated to helping artists:
There’s also TeePublic Blog with a lot of helpful tips about designing, selling promoting, and tagging.
And if you are already a TeePublic artist and are subscribed to their Artist newsletter you get useful emails about how to improve your store and get more sales.