First, clarification. This isn't a portrait. You're now doing technical photography. Get into the frame of mind that you're photographing gears in a machine for someone who needs to assemble the machine. Your artistic vision needs to take second place to this purpose. First and foremost, show the makeup. This is a huge mistake a lot of people make. Not that I have a massive amount of experience, but many very experienced nature/etc photographers make it - they wind up showing their own skill instead of someone else's. Get into the right frame of mind, and you'll be fine.
You want to minimize shadows on the face: Ring flash, close up, high key lighting, white background. Depending on the shot, strongly consider a shallow depth of field. Tilt shift lenses can also be useful to direct gaze immediately to the makeup.
The model needs to have amazing skin. Needs, needs, needs. You're trying to show the blending of the makeup, and the technical skill of the makeup artist. You're bringing out as much of the grain and minute detail as possible, to show how brilliant the artist is. If she's got bad skin, that's going to come through. It doesn't matter how attractive she is. If she doesn't have the skin, don't use her - no matter how drop dead gorgeous she is.
Both you and the artist need to get up in her face with a microscope to see if her skin will work for what the artist wants to do, and so you know where any weird spots may be. Give the model to the artist, so he/she can work on the model and get an idea of what will need to be done. Picking specific tones, foundations, etc. is very important, as different skin types will do different things in pictures depending on the type of makeup. Poor makeup choice under flash will cause horrible problems.
Do not work with hot lamps. The makeup artists needs to have full control over what kind of makeup they use; photography actually eliminates some kinds of makeup in the first place. Using hot lamps restricts choices further.
Do not photoshop aside from curves. It is not useful for a portfolio, as care must be taken to not remove the fine detail you've painstakingly captured.