Do your research. If you're not familiar with certain types of characters--racial groups, career types, etc.--then it's important to research what you don't know. Don't make assumptions.
Create three-dimensional characters. This means your characters should have equal strengths and flaws. No one is perfect and neither should your characters be.
Go against the grain of expectations. For instance, your gay character might be a champion bodybuilder. The blond cheerleader might also volunteer her services at a food bank. That gangbanger might be a college graduate.
Put yourself in your character's shoes. Stereotypical characters are often created because the writer doesn't relate to that character herself. Even if the character is incidental, imagine who or what that character might be and why she became that way. You don't have to write an entire back story for the character, but try to go beyond the surface and dig a little deeper. It will come through in your writing.
Read as much literature as you can, especially literature that is outside your own culture or genre. If you're not familiar with different writers of different cultures, ask your local librarian or bookseller. They will help you find authors that might broaden your reading experiences.