Double chins come in two flavours: the ones that can be tightened by moving the head a little forward, and the ones that need the photographer to change his point of view. Looking slightly upward is a lot more comfortable than trying to emulate a giraffe/tortoise Dr. Moreau experiment. (Okay, there is a third flavour -- the n-chin that is so prominent that it cannot be eliminated at capture time save for hiding it with arms, props or clothing. Those tactics are not going to work for executive headshots, but they are useful in glamour-style shots.) Raising the camera is often the best way to achieve a good picture without forcing the subject into uncomfortable contortions.
It's almost always a better idea for the photographer to be flexible than for the subject to be forced into some preconceived notion of what a good subject ought to look like. When you give minor direction, your subject will understand that you're trying to make them look their best. When the directions are headed toward forcing the subject into an uncomfortable position (and you're not shooting an "action" pose), you're telling the subject that there is something fundamentally wrong with their shape, that they have to significantly bend and twist themselves in order to look acceptable. That's not the message that comes across when you move -- it's like the difference between telling them you're looking for the best picture of them and telling them that they're really not a suitable subject, but you'll do the best you can (and thank goodness there's Photoshop). Your subjects should never even get a hint that there are hurdles to overcome, even if they are high enough that you need a personal jet pack to get over them.
Glossy skin has two basic causes: aging/drying effects (the skin becomes somewhat like parchment and actually has a smoother surface) and greasiness (which may be natural or may be due to inappropriate makeup -- inappropriate in the photographic sense, that is). Powder can help in some circumstances, as can wipes. Powder is probably more acceptable to subjects -- even male subjects -- they've all seen makeup done for movies and television and understand that it's all a part of making people look their best. And that's always the trick -- blame it on what the camera does and not on what the real problem may be. If you aren't comfortable/competent with the makeup thing and don't have a stylist handy, then handle the problem in post.