This is a commonly known one but the smell stays in your hands, and lots of people seem to forget that because they are so worried about their eyes and clothing.

If you want to exhale smoke without it being seen (such as you just got caught with a bunch of smoke in your lungs but they don't notice it yet), take off your shirt/sweatshirt and while its over your head exhale. As long as you try to not have any large holes in the clothing that it can escape from, then most of the smoke won't be visible.