Classic and modern men's haircuts all share the same idea—flat, lean sides in one length on top. What length you choose for your male client doesn't matter, but by keeping this shape in mind you’ll end up with a very masculine result. When you want to flatten or slim down a men's haircut, its best to work from the top to the bottom. Working from the bottom to the top tends to build weight through the recession area, creating a wedge shape that you’ll more than likely battle later. Here's how to create a hot, trendy undercut for…