oh gosh,
i suck at portrait, the informations that i have gathers from people is that if you are using available light, then take it near a window (assuming it's indoors) for example, here is a shot (very blurry, i was playing without a tripod)
The window was on his right and the wall behind his was tinted white.
There are some basics such as Lowest Aperture possible.
here is a link that i found great for windows lighting
http://super.nova.org/DPR/Window/
In terms of backgrounds, plain white is always the first pick, just make sure he isn't wearing white. you want it to stand out. and no complex background.
But i found that as long as their face is good, your 70% good.
i am not sure what kind of lense you use, when i was taking portrait of people, i always use the longest end of my lense for example, i had a 28-135 and most of the time i would use 135mm and stand far from the subject to provide that depth of field.
if you need to get a reaction shot, just talk to the subject with your eye of the viewfinder and the take photos while talking to them. i find that helpful sometimes, but it gets really weird facial expressino shots as well.
i think you know most of the stuffs that i know already, i just know some basics.
you can see of a section in my website with some portraits for a company.
http://www.yangphoto.com/1.html
most of the office shots were taken indoors with windows light, when there were window light, you can see in one of the photos with the orange background, i used my 580ex with a lumiquest diffuser (
http://www.lumiquest.com/lq952.htm) and i pointed straight at them. right now i am using gary fong's diffuser but they are pricey and not sure if it's worth it yet.
i suck at giving advice, just ask me questions if you have any.
oh yeah one last advice, if you wanna do window light, choose a window that isn't too big and small, around the size of the subject maybe alittle bigger will provide the brightest light. here is a sample
http://www.yangphoto.com/people-(10).html
everything is just my opinion.
good luck
mike, i hate portraits.