The best way is to hit the copycat where it really hurts: in their social currency. Speak out about the imitation; publicize it. Make sure everyone knows that your work is so good that it’s worth copying!
If you do this well, like Stevie Koerner of tru.che did last year when Urban Outfitters used a design similar to hers, you might even make a bundle off of being copied!

Gwenn: You say “I’m saying they should find new ways (ways that don’t involve copyright) to call out the copycats.”
Do you have any thoughts on what this might look like?

Barbara: How did you handle the student work that looked like your own?

Great reply, Mary – love your thoughts!

Thank you, Gwenn. I’ll read it.

The free culture movement does not push the idea that artists should work for free. Far from it, the movement is working to help artists find new models for making a living.
This is one such model, if you’re curious. Also, it’s probably a lot like what you already do, except that it involves acknowledging on a deeper level how indebted all artist are to the cultural commons.

When I am involved in the creative process, copyright concerns are not involved. I know very clearly when an idea is mine and when it is not; when I am copying and when I am not. We are are all influenced by what we have seen and by our culture. Being human is about connection. Our very being is an accumulation of our memories and experiences.
This morning I was looking at a wildlife artist. I noticed that while he painted animals, his primary concern was abstract design – much like Andrew Wyeth. I noticed that he tended to crop objects at the edges of his paintings in a seemingly random way – much like a camera cuts things off. I thought about how the camera influenced the Impressionist’s composition, especially Degas. I thought about how the wildlife artist’s use of texture reminded me of Japanese brush painting. All of these thoughts might influence the way I think about my next painting. I’m derivative because I’m alive. But, I don’t copy. The wildlife artist didn’t copy. If someone copies the painting I create, though, they’ve infringed on my copyright and we’ll have a discussion about it. It’s kind of like pornography. I can’t precisely define it, but I know copying when I see it. I’m not worried about any damage to creativity caused by protecting the artist. The human mind in infinitely creative. An artist isn’t limited by copyright. All he has to do is get to work and create.
Free culture? All of my thoughts this morning were free and didn’t infringe on anyone’s copyright. When keeping a roof over my head and food on the table are free, maybe we can talk about free culture. For now, I’m like any other working stiff. I want to be paid for my labor.

They aren’t separate issues. The remixing that Kleon and you all advocate is just as illegal as ripping another artist off under current copyright law since the law prohibits derivative works.
If you care about creativity, you have to care about copyright. They’re so linked! To learn more about how copyright law is damaging to creativity, I would recommend the documentary RIP: A Remix Manifesto or Lawrence Lessig’s book Free Culture.