(This has been the biggest procrastination and avoidance of work week for me, and I "blame" you guys for getting into such fascinating discussions that I feel inspired to contribute my two cents! 
How's that for avoiding responsibility?
Just a quick addition to my old post that Lyra mentions above about how to be invisible....the other piece that's really important to this to work - besides withdrawing your energy field - is to be STILL. Focus on quieting or calming/neutralizing your emotional and intellectual bodies. So you have emotional neutrality/non-reaction and you're not hooking into the emotions of other people as you walk along.
From a clairvoyant's perspective, there are tons of "umblical cords" or psychic cords of energy being thrown your way when you walk down a street or into a crowded building. I can see these cords as these little fleshy lines of energy coming out at me like ropes. Everyone's busy releasing lots of emotion, showing off, seeking attention, and throwing out these cords at you. If you're aware that this happening, you can practice neutrality and non-reaction. You don't need to be able to "see' this to know it's happening. It's enough to just be aware that people are trying to plug into you with various emotions and thoughts to get a rise or reaction out of you - and if you want to be invisible, you'll want to bypass these cords by not responding.
Avoiding eye contact is important, too. That's another big way people plug into you.
And also....just....try to be....SMALL.
All of these things will help you to be invisible even in potentially life threatening street encounters.
Conversely, if you want to get noticed by others, play the narcissist's game. This involves making your auric field as huge as possible so as to bowl people over with your energy. And make sure you put out lots of sexual cords and hooks to plug into anybody who is receptive, and try to get lots of loosh harvest off of people by stirring up their sex drives and their emotions.
All of this stuff works two ways. It's neat to practice BOTH sides of the energy thing, because then you can see how it works. I used to be a stage actress and learned the part about making my aura huge and plugging into people - mostly plugging in through the heart, because I wanted to express love and positive energies towards people. Nowadays I'm more in a hermit/writer stage, where I try to leave as little "imprint" as possible in my daily life around town. It works out well. Nobody really knows me or what I'm about (unless I want them to,) which means I'm not constantly picking up on low level, judgmental thoughtforms from people (about how I write astrology columns for a living, yada yada, and how I'm that weird psychic chick, yada yada. It's nicer when people don't know a damned thing about me if I have to bump into them at the store, etc.)
I believe many mystical masters have talked about the need for anonymity and invisibility to maintain your energy reserves - I think Carlos Castaneda wrote about this, didn't he?
Anyway, it's important to take your aura out for a test drive and get it to do all manner of things so you can understand how to shape and use energy. That way, you'll be prepared for all situations.
LipstickMystic aka Jennifer
PS I talk about about life and death street situations above....and this is no joke. I've had my share of death threats from physical stalkers and at one point was forced to live in extremely close proximity to two of them because the police didn't take my complaints seriously and at the time I couldn't afford to move. So that was interesting, walking by my stalkers on the street and them not even "seeing" me.
And in another situation - well, this was more of an instance of positive synchronicity or psychic shielding/avoiding violence, but it's related to being invisible....This happened while I was still living in Princeton, right around the time it seemed to...change...and perhaps become a hologram or overlay from the Matrix, because it didn't feel REAL anymore and had turned so negative.....anyway, I was headed to my office for an evening appointment and was walking down the street to my car....and not two minutes before that some bank robbers with rifles had hijacked somebody in a car right there and forced the driver at gunpoint to take them for a drive as part of their escape from the police....and there were no signs of this when I went to my car, I learned about this later. The person had been pulling out of the same parking lot where my car was, and had I been there two minutes earlier, I would have probably been the one who had been hijacked at gunpoint. Fortunately, they let the driver go and didn't hurt him, but still. It was a pretty "BIG" event for a town with relatively no crime - at the time. And I was very glad that my own sense of psychic shielding and self-protection was in place, because I think it prevented me from experiencing a negative synchronicity and aligning myself with that car hijacking. So psychic self-protection can work at several levels - making you invisible to chaotic people and helping you to unconsciously avoid chaotic events because you simply aren't attuned to them. You're on a different vibrational level where you are not willing to entertain self-damage.
LipstickMystic aka Jennifer
But yeah, there are definitely ways to deal with this sort of thing. Lipstick Mystic mentioned on another thread some months back her experiences with frequency shielding and "invisibility" in crowds, and so maybe you might want to start practicing doing that during the day, when you're out and about in the world. Play around with barriers on yourself, see if you notice people suddenly not seeing you, and when you get good I'm sure you'll be able to do that at night on empty streets or at the busstop, and watch those thugs pass right on by.....seeming to not even notice you. Cool stuff!
. Sending gratitude to you!