Eating a wide range of healthy foods that feed and support your microbiome, particularly soluble fibres which act as prebiotics. Focus more on feeding your microbiome than "adding to it" with probiotic supplements. Supplement companies fund and conduct studies in ways that make their product seem beneficial, but the truth is, we don't really know if those probiotics you buy on the shelf of your health food store actually colonise the gut, and whether those particular strains of bacteria are what your gut needs. Keep in mind that your gut contains 300-500 bacterial species, whereas probiotic supplements often contain only 1-30.