There's some good books you can read on diving, most notably JOhn Hoffman's The Art & Science of Dumpster-Diving. A pretty entertaining read even if you're not interested in applying John's lifetime of knowledge, but you probably will be after reading it! I'd like to offer some basic, quick lessons I've learned so far. I'm still an amateur at this, hopefully to one day gain the rank of Master Diver, but I have learned a lot quickly.
Where: Well, depends on what you're looking for. For us, we like food & books (more on food saftey later). So we target local markets that don't have the big trash compactors that I don't care to try to get into. I think that might count as breaking & entering. So smaller chain stores, and health food stores often have regular dumpsters. But whatever you're interested in or like to shop, shop their dumpster! It's usually a good idea to scout your location during the day.
When: After store hours, preferably at night for me are the best times. You could get away with looking through a dumpster during daylight store hours, but you risk being hassled by employees or managers. If they see you, you risk having them lock the dumpster or something. My favorite place is next door to a bar, and often employees come out the back door & can see me. I think they called the cops on me, but the cop didn't care that I was lookin'. Still, I don't like the hassle risk factor. Sunday evening they're closed, so I pick at my leisure that day! Ideally, I would go out after midnight every time, but I do work a 9-5 job. Gotta sleep!
Last night was good (Sunday!) Got 3 half-gallons of milk, 1 chololate!, lotsa yogurt, but some turned out skunky, 4 boxes of donuts, a little ice cream, broccoli, a few bunches of bananas, a few dozen small quickly-ripening tomatoes that I made yummy sauce out of today! Nummy! 6 cases of discarded books, too. Some light reading!