Final thoughts:
Many of the posts on this thread have been technical -- what rig, lens, etc. to use. Mastery of this is a given for great portraiture. Reach the point where you think of your gear little more than the carpenter thinks of his hammer.
Jerry, through his rules [First class exposition, Jerry!], covers posing the model(s) in detail. There's a wealth of distilled knowledge there for the taking. Reading and re-reading the rules is a good thing to do. Internalize them. And know when and, most importantly, why to 'break' them.
Lighting has been at least alluded to, and you can easily work it out for yourself by doing a bit of rummaging about on line and on this site. You can also go to almost any shopping mall portrait studio and check their set-up. Never forget that at the very least your work must measure up to theirs in technical and lighting competence. As you set up your 'studio' lighting, have a friend sit for you so you can gauge various effects and 'calibrate' your system to the final results.
[Anyone out there willing to do an article for TPF on lighting for portraits? I'll edit, if you want someone to check your sentences, punctuation, etc.]
That leaves me a bit of wiggle room to return to the thrust of my previous post [great portraits] and amplify it a tad. I'll add three more things to consider before posing and shooting.
1. Take time to get to know the subject if you don't already have at least a speaking acquaintance. A portrait should reflect the person inside as well as the surface appearance. Sometimes a simple prop will help in suggesting the particular over-all character or some unique feature of the sitter. Often it's worth while to spend 15 minutes or so just talking with the subject a couple of days before the sitting. You're after a great portrait, remember? Better than the mall stuff. Great has never come cheaply in time or effort. During an initial conversation you can suggest clothing to wear for the shoot, make-up, etc.
2. Find out who the portrait is for. If it's for the sitter's parents you'll want to shoot it differently from, say, a portrait of a young lady who wants to send the print to her boyfriend in the service. The differences may be subtle, but we're talking great portraits. Subtle is important. Subtle makes a difference.
3. Never be afraid to vary from the normal if the final portrait will benefit. A full-length shot of a young man facing away from you, jacket slung over one shoulder and looking back at the camera may be far truer to the character of the subject than the best possible head shot. Backgrounds other than the studio can enhance the final product no end if they are true to the character of the sitter. I can envision certain young ladies of my acquaintance who would be well-portrayed if photographed while in motion on a swing.
Regards, and enjoy both the work and the results of high-quality portrait photography.