I first started using affirmations several years ago and they are now part of my daily routine. I like them a lot because they're simple. They require little effort too which appeals to me greatly. It's the fast happiness approach.
There are some basic "rules" required to using affirmations. They have to be positive (of course). So you wouldn't affirm: "I wish I wasn't so tired". Better to say instead: "I have abundant energy." The reason for this is that your subconscious doesn't understand negative commands. If you say: "I am not a smoker", your subconsious will hear "I am a smoker" and you're stuck.
The other rule is that your affirmation should be in the present tense. If you wish for something in the future then what you're trying to affirm will always dangle before you, out of reach, like the proverbial carrot. So you affirm in the now. It doesn't matter if you don't believe consciously what you're saying. Your subconscious will take it in and start reprogramming how you operate. This is because the subconscious doesn't have a sense of irony or humour, and will interpret everything you say literally. That's why you have to be very careful what you tell yourself. I try not to use the expression "pain in the arse" for instance, for this very reason.
I first came across the affirmation method in the book You Can Heal your Life by Louise Hay. The most powerful and fundamental affirmation she recommends that you say to yourself every day is: "I love and approve of myself." It's more powerful if you say it aloud. It's even more powerful if you look at yourself in the mirror when you say it. What you're doing is taping over all the negative messages that have tended to become habitual. I work with a woman who is a wonderful person but I have heard her say so many times to herself when she makes a mistake: "Oh, you stupid woman!" We all tend to be like that, being our own harshest critic when we should be supporting ourselves.
Don't worry that you'll become arrogant or narcisstic in loving yourself. People who are like that are actually insecure and covering it up. By working on yourself through positive afirmations you'll actually come across in a more positive light to others because you'll feel better about yourself and people will pick up on that.
I worked with a recovering drug addict who used the Louise Hay book, telling himself over and over again: "I love and approve of myself," even though at the time, as he put it, he felt "like a sack of shit". Eventually though, this process helped him stay off drugs and he was able to start a new life. He didn't need to believe in the technique every step of the way for it to work.
The thing is to be patient, allow the process time to work, make it a daily habit and you will see changes.