Here is a good mantra tutorial and here is little more about them Collection ...DILD

1. should be short, but long is fine, if it flows naturally, you don't forget your line, it doesn't make you think too hard about what to say next
2. present tense
3. anything, that sounds and feels right and good to you will work
4. most effective when last thought before falling asleep
5. have the right attitude ("it didn't work, of course" - is not the right attitude)
6. more effective, when you make it rhyme and coordinate it with your breathing. Half of mantra on inhale, half on exhale, for example. Don't let it mess up your normal breathing speed and pattern.

Hope this helps.