With Prospective Student Athletes (PSA's) making earlier and earlier verbal commitment to college programs, the Unofficial Visit has dramatically increased in importance. I suggest that you take a few initial Unofficial Visits at the end of your Sophomore year to get a feel for the experience and schools, but you should really focus on visits once you get into the early days of club volleyball during your Junior year.
Back in the 'good old days' (which were actually rather nice), Unofficial Visits were taken just on a regional basis and usually when there was a strong connection with a school (alumni, team mate went there) - going out of state was rare, because of the time and cost considerations. A mid west school I was at, rarely had border state PSA's coming to campus for a visit - now, families are flying cross country to see a school at their own expense.
Because of the Internet age, information is more readily available to PSA's and sincere interaction between coaches and recruits is commonplace. Also, as the club volleyball system has become more of a business, these organizations are much more active in assisting families with the process. This has created a situation where the Unofficial Visit is now the in-person feedback on a potential school.
So, how do you go about an Unofficial Visit? First off, remember that an Unofficial Visit is very close to an Official Visit, with two exceptions; 1) The school may not pay for anything during your visit (except Division II which can provide one meal on campus), but you can spend the night on campus if you pay the customary overnight fee (determined by the admissions office), 2) The coaching staff may not have any off-campus contact during a visit - they can't give you a tour of the town or share a meal at a non-campus restaurant. Other than those two general examples, the Unofficial Visit is just like an Official Visit.
It is important that you treat your trip as a de facto Official Visit; take a campus and athletic facility tour, spend the night in the residence halls, eat a meal in the cafeteria, schedule a meeting with the Athletic Department academic person and the school's academic representative in the major you wish to pursue, watch the team practice and SPEND time with the players.
All of these items should be requested by you via e-mail or during a phone call that you initiate. Remember that an Unofficial Visit can occur at any time (outside of a Dead Period) and you can engage in as many as you wish. Be direct in letting the coach know what you wish to see and who you would like to meet with. Plan on your parents being with you, because having a couple of extra eyes looking around is a good thing. Once you have communicated with the program, they should have everything arranged for you and all set up. If the coaches are taking a relaxed or low key approach to the visit and have not fulfilled your request, then this should be a clear signal that they are doing this as a courtesy, in case their top recruits do not work out.
Spending time with the other players would probably be the most important thing you can do. If you have been interactive with the staff and done a bit of research about possible majors and campus characteristics, you should have a good feel about these areas. What you need to know is what are the other players like. If they are sincere and supportive during your visit, odds are they will be that way when you are a freshman. Also, you should be hanging out with Freshman and Sophomores, not Juniors and Seniors - the upperclassmen will not be there by the time you come onto campus. If you are not with underclassmen or those underclassmen are not good kids, then red flags should be going up.
With the Unofficial Visit being the visit before making a decision, you should time your trip to coincide with some type of training session. The non-traditional season (Spring) only allows for weekend tournaments/matches, that are rarely on a school's campus - ask to go watch a training session; if you are visiting a Division II school, you can be involved in the training session! By seeing a practice, you will get a feel for the training philosophy of the program.
The Unofficial Visit has dramatically increased in importance; gone are the days of a casual trip to hang out on campus. Now, you must be focused on details and aware of many different impressions - this is your one and possibly only opportunity to determine if this school could be your future!