I seemed to cap with 100-400/month with Zazzle.
Zazzle stopped their partnership with keds shoes, a product line I put VERY much of my time into marketing and by the time it was exponentially taking off they pulled the rug out from under my feet. I honestly have TOO MANY tips for zazzle and a half finished book on the subject. I changed direction and am furthering my knowledge of marketing before I hop back on the clothing train. It'd take forever to post everything I know here... feel free to ask questions
A few VERY quick tips:
The best way to make money with zazzle is to develop products for specific avatars. These are my top sellers (post-keds) and still make a few sales a week:
WASD SHIRT from Zazzle.com
Baby Stats Shirt from Zazzle.com
Police Line Tie from Zazzle.com
The first two are geared at geeks/gamers. The last one I envisioned for policemen balls and other functions cops would have to go to. The more clear the customer in your head while designing the better.
For 'artistic' stuff basically showing off a certain style of art, it's It's all about building your brand outside of zazzle. Use tumblr, twitter, youtube, facebook, and pintrest to draw traffic to your brand (always use an affiliate link) for extra money.
Some other shops I run passively:
Shop for Female Teen Artsy Peoples: ArtisticRomance: Home: Zazzle.com Store
Randomness: CherryCards: Home: Zazzle.com Store
For Comic book nerds: ComicSpirit: Home: Zazzle.com Store
Cat lovers: ThatCrazyCatLady: Home: Zazzle.com Store
I have many more designs from YEARS ago I never uploaded. I have a failed strategy in which I spent money to sell shirts that didn't work you can read about here: Experiment: Mayan+Twitter=Profit?