personally, i think asking a question about where something is or for the time isn't really doing much. maybe it'll get you to talk to strangers, but i really think you should go deeper. as soon as you ask where something is at the conversation is done and they're out of your life.
style had the best approach for moving targets. walk like you have somewhere else to be. walk past them...not so fast you seem unusual. (you probably don't want to approach from the front. that's usually a lot harder.) casually look over your shoulder and say, "Hi. I've only got a few minutes, I was here to meet some friends..." (which of course you is true

) "...but can I ask you guys for your opinion?" Then spit somethin' out and go w/ it.
If that seems like a little too much, try an easier target. I think older women are perfect. These are the people you can ask about where something is at or just talk about the weather as you wait in line for the bathroom.