Basic Tips To Avoid Tokenism
Tokenization is essentially when a minority character exists simply to have a minority character. At best, it's a rather weak attempt to be inclusive; at worst it's a half-hearted attempt to quiet critics. Either way, it's something that's best avoided.
Include several minority characters.
One of the surest signs you've got tokenization going on is when you have a cast full of non-minorities except for one or two people representing minorities. If you have a cast with a bunch of white people with one black guy or a cast with a bunch of men and one woman, you probably have tokenization going on. As a bonus tip - when trying to figure out how many characters of non-white races to include and which ones, try looking up the ethnic demographics of the place the story is set in.
Make sure the minority characters have important roles in the overall story.
If your minority characters limited to sidekicks, secondary characters, and/or plot devices, you're doing it wrong. Make sure at least some of your minority characters play important and key roles in the story and do more than simply help a non-minority reach xir ultimate goals and/or glory.
Your character's identity and interests should not revolve around xir minority-ness.
Your character's minority status should be a part of your character's identity, but xir identity should not revolve around it. Your character should have plenty of hobbies, interests, life experiences, ambitions, and goals that any other person could share.
Pass your Bechdel Tests.
The original Bechdel Test runs as follows: does a film contain at least two female characters who talk to each other about something besides a man? Similarly, does your story contain at least two non-white characters who talk about something that isn't racial issues or a white person? If you can answer "yes" to these or any minority group represented in your story, then you're probably a fair step away from tokenism.
More Resources/See Also
Basic Tips To Get More Racial Diversity In Your Writing
Fangs For The Fantasy (offsite)
Ars Marginal (offsite)