How to use it, and how not to use it.
The word “myself” is what's called a reflexive pronoun. Think about looking in a mirror and seeing your reflection. You'd say, “I see myself in the mirror.” You see your reflection, and “myself” is a reflexive pronoun.
Other reflexive pronouns include “himself,” “herself,” “yourself,” “itself,” “themselves,” and so on.
You use reflexive pronouns to refer to the subject of a sentence again, later in the sentence. For example, you could say, “I see myself playing marimbas,” or, “I'mgoing to treat myself to a mud bath.” In both these cases you are the object of your own action; the subject is “I” and you use “myself” to refer back to that “I.”
If you're an astute listener, you will have noticed that “myself” is in the object position in the sentence “I see myself,” but I said earlier that “me” was the object pronoun.
Here's the rub. You use regular object pronouns when the subject and object are different, and the reflexive pronoun when they are the same. That's why it's right to say both “I saw him,” and “He saw himself.”
In “I saw him,” the subject and object are different people, so you use the object pronoun, “him.”
In “He saw himself,” the subject and object are the same person, so you use the reflexive pronoun, “himself.”
Again, it can help to remember the reflection analogy for reflexive pronouns. You have the real person (the subject) and the reflection (the object that is a reflexive pronoun).
“Myself Is Also a Intensive Pronoun
Reflexive pronouns can also be used to add emphasis to a sentence. (In case you care, then they're called intensive pronouns.) For example, if you had witnessed a murder, you could say, “I myself saw the madman's handiwork.” It's dramatic, but it's also grammatically correct.
If you want to emphasize how proud you are of your new artwork, you could say, “I painted it myself.” Again, “myself” just adds emphasis. The meaning of the sentence doesn't change if you take out the word “myself”; it just has a different feeling because now it lacks the added emphasis.
There you go! The quick and dirty tip is to think about how you would write the sentence if you were the only one in it, and then use that pronoun. For example, “Please contact me.” You can also remember that it's OK to use reflexive pronouns for emphasis and when the subject and object in the sentence are the same.