Pick up an editorial / whatever magazine you have handy and look at the images. Yes they are most likely horrible just remember somehow this stuff sells. Anyways... Look at the images specifically the portraits. I want you to guess the focal length and aperture. To be honest after looking at so much exif my self I can usually call it. There are things that give it away as far as focal length go.
Look at the lines in the image, the straighter they are the further away they were shot. Most likely the mag will not have anything wider than 50mm probably even 85mm.
Aperture is the one that is more fun. Its quite easy to guess f/2 but it gets quite a bit trickier when you get around f/4 - f/8.
If you know these and can pick out these images it will help you obtain your specific creative visions. If you know you want a certain look it will help you create it.
I hope this was a good warm up for you photographers out there :)