Make Your Boundaries Clear
Ask any parent and they will tell you that once a promise is made or a boundary is set, there must be follow through. If you say "only two cookies" yet in the end allow five to be consumed, children will know they can challenge your word and succeed.
Those you are leading will do the same. Flimsy expectations or wavering requirements create a loose working environment. Not relaxed, but one where people question their standing and non-driven employees will slack.
Make your boundaries and expectations clear, and reinforce them constantly. Leave room for grace, but not doubt.