Win a Contest
This is probably the hardest way and you may need some luck to get free tickets this way. Either you have to win a contest or be lucky to win a ticket through a draw where every entrant has a chance to win tickets. Cinemas as well as movie promotion companies often offer both kind of options and you can get free tickets this way.
If some friends are going to the movies, tag along with them. You may be an unwelcome guest but you will surely be able to see the movie for free. When the time comes to buy the tickets, you can slip away for a little bit. It is the perfect way of watching one for free. Though make sure that you do not over use this tool as you may get the boot eventually.
Work at the theater
It is a wonderful way to make money and to see movies for free at the same time. They offer specials for the employees and since you are working there, they would probably not mind you watching a movie when you are off and the film is screening with a dozen of occupied seats.
One of the most fun jobs that you can have is to be mystery shopper. Go to a movie, buy a ticket which you will be reimbursed for and watch the movie. Afterwards, make a report about the service of the cinema and get paid for it. This is the best way to watch a movie for free and even get paid for just filling out a form later on. One of the many simple joys of life.