know which side of (one's) bread is buttered
To know how to best ensure one's success. I know which side of my bread is buttered, so I was very nice to the recruiter and promptly sent her a thank you card after our interview.
know which side of one's bread is buttered
Be aware of where one's best interests lie, as in Jerry always helps out his boss; he knows which side of his bread is buttered. This expression alludes to the more favorable, or buttered, side of bread and has been used metaphorically since the early 1500s.