Start by making sure your site is litter free. I have yet to be on a site that didn't have a trash bin readily available. There is no reason for the disposable cups and containers to be on the ground.
Keep materials stacked and organized. Not only does this look good by passers by, but it's a highly efficient work habit.
Keep barricades, fencing, and signage properly placed and maintained in good condition. Your signs are your representation, make sure you represent well. This includes flaggers and security, or anyone having interface with public. They should be attentive and courteous. Keep in mind, anyone walking by could be a potential client.
In short, we're not just "construction workers", we are business PROFESSIONALS.