I'll take a stab at this...
Most places will house you with a current grad student, which is great, because it provides you with the opportunity to gather a lot of info about the culture of the program, etc. On the other hand, for the anxious ones amongst us (speaking from experience!), it can make the whole two or three day affair feel like one long interview, because you are always being evaluated, either formally in an interview or informally by the person you're hanging out with. Faculty at most schools do gather opinions from their grad students, so if you are acting aloof, arrogant, or unpleasant during the down-time it is not going to reflect very well upon you. Impossible as it may seem, try to seem fun and relaxed while remaining appropriately concerned and reasonably professional (i.e., try not to get plastered and do something embarassing!). There are normally a bunch of social events like parties, dinners, etc. with the grad students.
In terms of the actual interview process itself, it generally occurs over one or two days. You will have a whole bunch of meetings with various faculty members-- at most places I met for 30 or 45 minutes with nearly every clinical faculty member, so you really make the rounds and it takes all day. The interviews themselves were more laid-back than I expected. They generally asked about my previous experiences, research interests, and plans for grad school. Be prepared to ask LOTS of questions about the program! In my very first interview, the prof simply said, "What can I tell you?" and had me ask him questions for 45 minutes, which he answered, before asking me what he could tell me again! He didn't ask me a single question. That's unusual, but really-- you don't want to sit there insisting you don't have any questions about the program. Make sure you come up with something. I was not really asked any tough questions at any of my interviews. (Well, there was one, but the prof emailed me about it and told me in advance that he would ask).
I was comfortable dressed casually and inconspicuously, in a suit. Keep it simple. Wear some shoes you can walk in (that also look halfway decent)-- one program had us walk 2 miles to a prof's house for dinner on the interview day, and I was NOT happy in my cute heels! Bring at least one other semi-dressy outfit (e.g. nice pants and a dress shirt), as well as some casual clothes.
The main thing is to come across as a fun, relaxed, confident human being. At this stage, you need to get people (including faculty, grad students, and other applicants) to feel like they want to spend the next 5-6 years with you. Do not be the person obsessively discussing their GRE scores at the party! Heh heh.
Sorry this was so long-winded. It's sort of fun to reminisce. Hope it's helpful. Oh, PS-- this applies for research-oriented clinical psychology PhD programs. I'm sure things are very different in programs with a more psychoanalytic approach, or what have you.
