This is something everyone should do and is done in some form by most successful people. IMO How effective it is depends on how specific you are.
Basic rules:
1. Be personal...always use 'I' statements
2. Always use present tense...I am...not I will be or I did
3. Be positive...never use the word 'not'...I will not be.. etc.
Using 'not' brings to mind the thing you don't want.
'I am healthy' or 'I am calm' is much better than 'I will not be ill' or 'I am not nervous'
Try saying them and feel the difference.
Repeat regularly, especially before sleeping and on waking.
The more specific you are to your own circumstances, the better the result.
Muhammad Ali conditioned himself to amazing effect by this method simply by repeatedly chanting the same four words.