Depends on the situation... While with my friends I'd say whatever suits me best at the time, from regular "cześć" ("hi") to "elo" which is an equivalent of "yo".

But when it comes to everyday traumatic situations such as walking into a shop, greeting professors at the University I'd rather not say anything at all. I'd usually do a XIXth century feminine bow (which with my coordination looks like trying to stand on one leg) and bow my head a little.
And run.

I say, "hey" "hello" "hi" to people in general. And to my coworkers "sup" with a head nod. haha.

oh hey

"Friendship is unnecessary, like philosophy, like art, like the universe itself. It has no survival value; rather it is one of those things which give value to survival."-C.S. Lewis<a href='<a target='_blank' rel='nofollow' href='http://www.squiby.net/level/5801630''>http://www.squiby.net/level/5801630' target='_blank'><img src='<a target='_blank' rel='nofollow' href='http://www.squiby.net/view/5801630.png''>http://www.squiby.net/view/5801630.png' border='0' alt='User Posted Image'>