A program like YCombinator or TechStars is a giant boost up for exactly that reason. But if you have to start at the bottom, the process is exactly the same... just longer.
It's like that guy who traded one red paperclip for something better (and better) over and over again until he had a house. Find the 10 smartest people you know, make friends, do something for them, and then ask them to do something for you: "Introduce me to two other smart people like you". Before you know it, you'll have some mentors. And if you keep it up, you'll get better and better mentors.