"In English, we use personal subject pronouns out of necessity, for nearly all sentences without subject nouns would be incomplete without them. But in Spanish, such pronouns are used primarily for clarity or emphasis, since a verb alone can be a complete sentence."
"Because the verb form often indicates who the subject of a sentence is, one can properly leave out the subject pronoun, or put it at various places in the sentence. "Voy a la escuela", "yo voy a la escuela", "voy yo a la escuela", and "voy a la escuela yo" are all grammatically correct ways of saying "I am going to the school" (although the final option would be uncommon except if said for poetic effect). But the placement of the pronoun can make a difference in how the sentence is understood."
http://spanish.about.com/od/pronouns/a/subject_pron.htm
"Subject pronouns in Spanish are a lot like medicine — they're often essential, but their use should be avoided when they're not necessary."
"Overuse of subject pronouns — the equivalent of words such as "he," "she" and "they" — is common among English speakers learning Spanish. It's important to remember that in Spanish the verb forms often make subject pronouns unnecessary, and when that's the case the pronouns shouldn't be used unless there's a reason to."
For examples where you have to use subject pronouns, see : http://spanish.about.com/od/pronouns/a/subject_pronoun.htm