What not to do:
Don't bother drawing every single line on the nose. That'll just make the picture look busy and amateur. Just draw the bottom of it, describing the end of it and the nostrils, and throw a line in there that shows how the bridge connects it to the forehead.
Here's a few different ways to draw teeth. I use the first if I'm in a hurry, the second if the subjects' teeth are important to their likeness. I only use the third and fourth I'm trying to make the subject look bad.
Some folks have a very turned-up nose. This example, on the far upper left, is generally as far as I'll go with that. This is one of those things that if you push it too far, they're going to look bad and you might not get paid.
If someone has a huge nose, you can split the difference between that and the average-sized nose that you've learned how to draw by now. So you draw a nose that's half way between the persons' actual nose, and an average one. This means you can still get a likeness, without offending them. The same goes for the mouth, head size, eye depth, everything. Find that middle ground to make (almost) everyone happy.
Of course, if you want to skewer everyone, take that big nose and make it bigger. Same with the mouth, head size, eye depth, everything.