Okay then.
#1 Go at night (for the obvious reasons -- fewer people around to harass you)
#2 Be quick - Don't hang around forever
#3 Tech stores are good - More expensive trash. I suggest retail places like compusa,circuit city,best buy,etc.
#4 Once you find a good place, check it at least once a week. They aren't always throwing good shit out but you can hit it pretty big if you're patient.
#5 If you find a place that locks their dumpster, don't even bother going back again. Very little chance it will ever be unlocked.
#6 Watch out for cameras, it's better if you know about the place a bit. Cameras can fuck you.
edit:
#7 Don't go at the same time every time you go. Try to go early and then go late, skip a day, come back, whatever.
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Final thought:
It is illegal, if people are really anal, they can send you to jail. It is "technically" their property or the disposal company's property. So just follow those rules and don't leave a mess (i.e. throw shit out of their dumpster and leave it open or whatever) You can make out pretty good.