I always make sure to take my 3DS when I go to the mall & stuff. Also a lot of my friends play acnl so I get the better items from streetpassing them tons of times.
I've only street passed once (the second time the game fell out of the 3ds) and I got loads. I was in a city, it was half term so the kids were off school and I stopped at a couple of comics shops so who knows which is the most contributing factor ahaha!
I seem to have done not too badly streetpass wise, I'm in the UK and if I take my 3DS with me on a shopping trip in town I usually get about 8 people which is more than the 0 i expected since no one i know has a 3ds ahah.
Did take it with me to London and got 20 really quick, just didnt realise then to clear them to get more!
Annoyingly now I'm down to 9 villagers again so I dont really want to streetpass and get another voided villager.. but I want those badges. Hmm!
It depends on the country, I guess. Like it's easier to get Streetpasses in Japan compared to a minor country like New Zealand. I've only gotten 3 Streetpasses but only one of them was for New Leaf. The other one was for Mario Kart 7 and then the last one I just picked up randomly.