The best way to someone in the contemplation stage of change is to

When someone is in the contemplation stage of change they are weighing up the pros and cons of stopping the bad behavior or modifying it. They are undecided yet as to what to do, they are thinking about their options and this can take a long time. During this time though they are open to suggestions and tend to be more receiving to information from others as to how their behavior is effecting them and others, so I would suggest that the best way to help them would be to talk to them about their behavior, make suggestions in a non judgmental way as to how they could stop or modify their bad behavior, be there for them in a positive and informative way. You need to help them recognize how bad the behavior is for them and reinforce that ultimately they know what is best for them.