Which ingredients should I avoid if I want to show my skin microbes some love?

Avoid products that will drastically alter the pH of your skin like soaps or other products with a high pH, or products with an extremely low pH such as chemical peels. Having an environment too alkaline and too acidic will create a less-desirable environment for microbes.

Avoid anti-microbials (preservatives and other) since they kill not only the bad guys, but the good guys as well.

Be wary of harsh emulsifiers and surfactants as these can often remove natural skin oils that are the primary food for your skin microbiome, disturbing the natural balance.