Learn all about the object of his adoration - "In war as in war", as they say, so for the time necessary to become a spy. You need to know as much as possible about the person whose attention you want to achieve.
Think about why did this man does not pay attention to you. After all, the answers can be absolutely unpredictable. Maybe he is married or he already has a lover, or he does sexual orientation. Situation may be different.
If, nevertheless, man is free, and still does not pay any attention to you, it's time to act. First you need to become good friends. At worst - friends. The main thing that people began to trust you. Then you will learn more about his life.
Interested in music he listens to, books that cause him admiration. Look at those films that he loves. If you will develop common interests and tastes - it can become a breeding ground for budding relationship.
Change the image. This is a favorite part of the female half of humanity - a new haircut, new clothes, beautiful manicure. Work out, give up the night trips to the fridge - and soon you will look delightful.
Learn about his hobby. If he is a football fan, go with him for the next match. If he's crazy about billiards or bowling, invite him to compete. The more time you spend together - the more chances to win his heart.
Admit your feelings. There is a category of people who absolutely do not see sympathy on the part of the opposite sex. Maybe your feelings will become a big surprise. Or will he also would not mind to answer you in return.
Do not be afraid to be funny. Men do not like girls who are offended or upset for any reason. But the sincerity and ability to be always in a good mood - these are the qualities that are priceless.