Have a game plan.
Make sure you're clear about what you're looking for and what you'd like help with. The reason these people are successful is because they don't waste their time on stuff.
Don't expect it to be a one way street. Offer something in return even if it's just time.
Listen more than you talk.
That's about it. Good luck!
For instance if a professor wrote a paper about topic X, look them up in the university directory and call them up, odds are pretty good they are in the office, then say you read this paper and thought it was interesting and you have this question and probably you will get an answer.
It really is that simple.