Positive affirmations, like a lot of really good self-help techniques, went through a period of huge popularity, and then were gradually overtaken by other, newer, and (apparently!) more sophisticated ways to go. Fact is, of course, that neither 'newer' nor 'more sophisticated' necessarily equates with 'better', and using positive affirmations - which are easy to construct and use (and FREE!) - is worth turning into a habit.
Positive affirmations can take several forms, but probably the most useful are the 'I am', 'I can' and 'I will' types that are statements of self-belief.
'I am' statements are statements of who you are, positive affirmations of real traits and abilities that exist in you and can be nurtured and improved. For example: 'I am competent', 'I am strong', 'I am talented', 'I am energetic', 'I am a good person', and 'I am caring', are all 'I am' statements.
'I can' statements are statements of belief in your potential to change, grow, and help yourself. For example: 'I can gain confidence', 'I can be strong', 'I can be assertive', 'I can be positive', 'I can change', 'I can heal', or 'I can be a problem solver', are all 'I can' statements.
'I will' statements are statements of positive change - success prophecies, or positive affirmations of things you want to achieve. 'I will take care of me today', 'I will feel good about me today', 'I will like myself better each day', 'I will only take on what I can handle today', and 'I will smile more today', are all statements of an intention to achieve something.
Positive affirmations are easy to construct, but they are more easily accepted and assimilated by the unconscious mind if they are short, specific, have strong emotional content, and address only one subject at a time - Keep It Simple Stupid, in other words.
Frequent repetition of positive affirmations will impress your unconscious mind and trigger it into making change happen for you, but you shouldn't expect instant gratification - and because affirmations need to address one subject at a time repetitively, you obviously you can't use more than one of them on any given day. In fact it's best to stick to a single affirmation for at least a week before moving on to another one, even if the subject matter is similar or, for that matter, identical.