I was tempted to do the much longer 'don't' list.  You know, the obvious stuff like don't come in reeking of booze or weed or otherwise obviously hung over - no sunglasses, no hoods down, no pajamas, no headphones etc. But who wants to be negative at the start of another year?  So let's stay positive: how to impress, rather than how not to avoid the 'merde list'.

            1) Arrive on time.  Nothing sticks a prof's mind more than the faces of those folks who roll in 10 minutes late and disrupt everyone as they find a seat and get settled.  Granted, there are times when it's unavoidable.  If that's the case, have the good grace to look apologetic.  Mouth a 'sorry' and be sure to explain to the prof at break or after class why you couldn't avoid being late - class on the other side of campus etc.

            2) Sit in the front half of the room if there's space. This is pretty obvious, but you should know that scrambling for the back row in a room where there's lots of space toward the front sends a very clear and very specific message about your commitment to the class.  This is very important in small rooms but also applies in larger rooms.  Even if a prof can't learn everyone's name, she will get to know faces.  And that can help if you ever plan to ask for help, or an extension etc.

            3) Look prepared.  Have your texts, notebooks, writing implements, and devices.  Get them and yourself organized before class begins.  This sends a very positive message about your professionalism.

            4) Be active. This begins before class - once you're set up, engage a classmate in conversation, bring some energy to the room.  And when class starts, look up, project interest.  Even better, ask a (relevant) question or attempt an answer when the prof (inevitably) asks a question.  Trust me, there's nothing more welcome than a raised hand when you're standing at the front of the room with a question hanging in the air as seconds pass.  The prof probably doesn't care if the answer is exactly what he was looking for. He will be able to use whatever you've said to move on, which is really want he wants at that point.  Your classmates don't care either - they'll be thrilled to move past the awkward silence.  And you'll be the one who saved everybody.  That makes an impression.

            The golden rule with all this is to remember that everything you do from day one sends  messages about your attitude to the class - and therefore to the professor.  This might seem strange, but remember that your profs have dedicated a frankly ridiculous amount of time and energy to their academic discipline.  For most, it is a central part of their identity.  Show contempt to it and you might just as well just flip him/her the bird.  If profs were entirely rational and dispassionate, none of this would have any impact on their impression of you or your work, but let's be realistic, they're human.