If you need to wear business clothes during the day, you won't need many casual clothes (a great source of wasting space, I always bring clean polo tops home after a trip). Pack 2 pairs of shoes max, so make sure they co-ordinate (green heels / red brogues depending on sex are not the best). Most hotels provide hairdryers. There is a big difference between taking a gym kit in case you might use it (you won't) versus because you work out regularly (you will). If you need warm clothes, try and use higher quality ones that are not as bulky, and wear them onto the plane rather than pack them.
Remember you might by things on your travels. I take a small folded bag that can be checked in on the way home with all the extra stuff (but check the airlines restrictions on how many bags you can check).
As jno says, the bag is critical and it is worth investing in (but not too much, 'worldproof' metal cases still get trashed). I have three different bags, depending on the nature of the journey. Compartmentalised ones are great if you have frequent stops en route as you don't need to unpack, but weigh more.
Regular flyers work out when they need to sleep and plan the journey activities round that. Consider eating before you get on the plane, so you can sleep longer. I read a lot on planes when I am not sleeping.