It's tough to be happy, joyful, successful, etc. if you don't think that you are a happy, joyful, and successful person. Think it first, then do it!
There's no arguing with this one -- research has shown the smiling has both psychological and physiological effects. So, put a smile on your face and you'll be on your way to a change in attitude!
Read books, articles, magazines that help you understand and adopt the new attitude. Watch films or listen to music that inspires you and encourages you to change.
It's hard to change your attitude if you keep doing the same old stuff the same old way. Do things differently to start thinking differently.
Make your environment reflect the attitude you wish to have. Create the physical spaced that makes you eager to change.
Find someone who already has the attitude you wish to have. Follow their lead, learn from their example.
One of the fastest ways to change your attitude is to take the focus off yourself and to help others in need.
Let everyone know what you're doing and enlist their support to help you change and give you ideas. The more you feel like you're part of a group effort, the more likely you are to be successful.
If the change you desire to make is a big one or is extremely radical, consider getting the help of a mentor, counselor, or coach. These professionals can reduce the time & frustration involved as well as provide you with many new ideas to help you grow.
Recognize that most changes occur slowly, over an extended period of time. If you don't get immediate results, don't be surprised and DON'T QUIT! Keep working, it'll come.