Men frequently agonize over the ideal of 'business casual'--and it's often obvious when we see full suits on Friday, or dark denim which is not appropriate. The correct answer varies but, there are a few basics to have to ensure you can go from business or casual to business casual.
(1) A sports jacket with minimal or no print to avoid the Professor-look; (2) light sweaters preferably v-necks as they add to the openness of the look and work well with tees (casual) or dress shirts; (3) khakis can be mature just get them in flat-front to avoid the 'fatherly' appearance of pleats; and (4) button-down shirts; I am a fan of cotton broadcloth because they're softer and less stiff-looking but, the choice is yours!
Some basics will take you week to week, and allow you to stretch your suits more appropriately!
FYI - question has come up about corduroy. Blueprint believes it can be done casually and even for business attire as long as you're not an attorney presenting in court or a business executive pitching to the Board. Stick to a thinner wale.....