Depends on what you want to change, exactly. I'll throw in some ideas here:
- For going to the gym, find a group of at least two other guys who will go with you. That way, you'll always have someone egging you on to lift.
- If you want to start eating better, just learn how to cook. Cooking gets really fun when you get into it, and just about anything you make yourself is going to be a lot healthier than just about anything you'll buy.
- For more generalized stuff, a routine is usually the best way to go. Ex: If you want to get up early, set a list of things that you want to complete by a certain time in the morning, which will naturally establish mental 'consequences' for not doing them. Habits are the best way to go.
- In general, human brains aren't that far removed from animals'. You can 'Pavlov' yourself into feeling certain ways about things, and that can definitely be used to your advantage. Positive and negative reinforcement are great ways to trick your mind into liking/disliking something.