Recognizing for whom you're writing is very important.  Your audience determines how you approach your subject.

The audience's age, gender, interests, socio-economics, education, intelligence, politics, religion, expertise, biases, opinions, etc. all help determine how you write. For example, you wouldn't write the same way for a group of fifth graders as you would for a college professor...unless he or she has had a very bad day.

Typically, your instructor is your audience in a writing class even when he or she says write for an audience of ___________.  Consequently, you have to meet their standards and appeal to their tastes.

Never think that you're writing for yourself unless you're writing a journal or diary that no one, besides a snooping sibling, will see.

Persuasive Writing and Audience

If you're writing a persuasive piece, you have to gauge your audience's opinion about your thesis: do they agree, do they disagree, or are they neutral?  This determines the type of support you will use in your writing.

Types of Support
Logos Facts, reasons, examples.  Logos, a Greek word, looks like our word logic for a reason.
Pathos Emotion.  Emotional appeals to a reader or listener's heart.
Ethos This refers to a speaker's credibility.  If they are perceived as being an authority on a subject, their audience will agree with them more easily. Most of us aren't experts, so we quote experts and leech of their ethos.

Audience Type Type of Support to Use in Persuasion
Agreeing When they agree with you, you don't need to throw a lot of reasons at them.  You can mention your main ones, but since you're really a cheerleader for this audience, you don't need to use a lot of explanations and example. Using pathos to get them fired up even more is your prime type of support. To reinforce and increase their agreement
Disagreeing When your audience disagrees, use a combination of logos and ethos.  I typically rely more on logos because I am more easily convinced by reasons and examples than emotional appeals.   However, the best you can hope for when your audience strongly disagrees with you is for them to take a second look at why they feel the way they do. 
To prompt them to reconsider their opinion
Neutral This is the type of audience I prefer.  I can  rely primarily on logos, but I can throw in a little pathos for the icing on my logos cake. 
To get them on your side
For all three types
Use borrowed ethos from the experts.  Be aware that both sides can find experts that agree with them.  Your expert needs to have the greater ethos by having greater expertise and experience on the topic.  One logical fallacy is using a well known person to promote an idea that they have no expertise in.  This is a common occurance in commercials.