I think the concept is at least debatable: since we don't go to this length for any other condition, does pre-test counseling actually add to the perceived stigma of HIV? Rather than prepare and educate the person, does it not then sometimes make the person less likely to cope? I'm not sure; it probably is a good thing for most people though.
Anyway, a number of you have written to me over the past few months asking about what to say in such an instance. It's been a long time since I've had to do much proper pre- or post-test counseling (we have professional counselors at our hospital who do most of this), so I've scoured the interwebs for a nice summary.
In my opinion, the single best source is available here. It's got three summary boxes (one each for pre-test counseling, and post-test counseling for both a positive and negative results) that contain the essentials, and it supplements this with a lot of useful background information to flesh things out.
I would add a few things, however. Firstly, with the pre-test counseling, I would personally go into more detail about what HIV is - many people are surprisingly ignorant of this common disease; lots still think (for instance) that you can get HIV from sharing a toilet seat with an infected person!
Also, if it comes to delivering bad news, I wouldn't leave almost everything to the specialist, as these guidelines suggest. By all means, refer to someone experienced in managing HIV, but if you are confident with the basics of HIV management, I'd spell them out briefly, so that the patient gets a first idea of what is in store. Also, always end off each counseling session by asking if that patient has any questions - you'd be amazed at how often they're missing an absolutely crucial piece of information that they were too afraid to ask about.