Let your actions take the lead.
Reflexively laughing at an offensive joke.
Thinking, "That overweight person shouldn't be eating that giant dessert."
Adjusting your posture or walking speed when you walk past a young male of a different race at night.
Treating male and female children differently.
Having a discomfort reaction to gay men displaying signs of affection, or to people who are transgender.
Some people are explicitly racist, sexist, classist, and/or homophobic and don't care. However, implicit bias is a huge problem too. Implicit bias refers to sneakier attitudes or stereotypes that we hold but are not generally aware of. Your explicit belief might be that everyone is equal, but you may find yourself reacting inconsistently, as illustrated by the examples above.
1. Take the Implicit Associations Test.
The first step to changing your implicit biases is acknowledging that you have them. You can check your level of implicit bias by taking one (or several) of the Implicit Association Tests on Project Implicit. You will answer demographic questions before the test; the entire process takes around 10 minutes. There are various test options including race, sexuality, and weight. The weight bias test is especially interesting because bias against people who are overweight is one of the few biases that remains at least somewhat socially acceptable. But the bias matters—e.g., a hiring manager may be more likely to hire someone who is not overweight because they perceive people who are overweight are less energetic or less focused.
2. Identify situations in which your implicit biases impact your behavior.
Make a list for yourself similar to the one of examples at the top of this post. Be as specific as you can. For example, you bake cookies when you're babysitting your niece but play basketball with your nephew. Make plans for alternative behaviors that run counter to your biases, like planning to bake with your nephew next time he visits.
3. Make an effort to be friendlier and act less threatened when interacting with people you perceive as different.
Behavior influences thoughts. If you act friendlier toward other people, over time you will feel more comfortable with them. Brainstorm some specific plans; maybe there is a neighbor you never talk to because they're older or have tattoos. If you're a college student and are more likely to ask questions of your American professors than your foreign-born instructors, maybe you decide to visit those professors' office hours more often.
4. Become aware of your "positive stereotypes."
Positive stereotypes may seem harmless but they can register as and feel offensive or restrictive to those on the receiving end. No, your lesbian friend probably doesn't want to help you assemble your Ikea furniture, and your Asian friends may not be the best people to ask for computer help.
5. Hang out with people who have better attitudes than you.
My spouse, for example, volunteers with an organization in which she works alongside homeless men. I get to hear stories about these men and it helps me to have a better attitude towards homeless people in general.
6. Expose yourself to media that aims to break down prejudice and discrimination.
One way to do this is via friends who post that type of content on social media. I have a few Facebook friends who are awesome at this and who routinely expose me to information I wouldn't normally see.
There are lots more practical tips in my book and in my other PT articles.