BONUS TIP: Know the difference between mistakes and carelessness--your audience does.
Mistakes here and there are forgiven, but if you think you don't have to proofread your work, you are mistaken.
You want to respect your audience--proofreading shows you care. You don't have to be perfect, but be as close to it as possible.
Each mistake, especially careless errors on the page, is a hit to your credibility.