As human beings, we all must make assumptions based on past experience. Making judgements about others isn't wrong, it is necessary to our survival. If, as a child, the first dog you ever saw bit you, you would naturally fear dogs, even if every dog you met after that was friendly and affectionate. Likewise, in our first meeting with any stranger, our minds apply past experience to make a decision as to the level of current danger. The first thing the mind brings up is past bad experience, which generates prejudice. Here's how to recognize and rise above unconscious biases that may be causing problems in your interactions with others.
Think about the phrases you use regularly or reasonably often. Have you ever said something like "She's sooo blond!" or "That's just gay"? Both imply negative assumptions! How about "Boys will be boys," which implies that boys can't behave better, should be expected to be crass, cruel, and careless, etc.
Make a list of all the phrases you hear that imply negative things about a category of people, whether they are based on gender, appearance, religion, politics, etc.
Now replace the general category with your own name. Instead of "She's so blond," see how it feels to have that judgement applied to yourself - "I'm so blond." Now think about people you know (or know of) who have that characteristic and are not "stereotypical" of the trait. Martha Stewart is blond. She certainly isn't stupid. Maybe your cousin Joe is blond and graduated top of his class at a prominent law school. Note all the exceptions to the genralisation you can think of.
Do this with each generalisation. Replace the main character with yourself, and see how the assumption feels. Find positive examples of each maligned group.
Whenever you find yourself thinking "ALL" of a group are something, pay close attention! No group is "all." Some are many, but even within the "many" there are vast differences.
Sometimes, our biases over-rate certain groups. Maybe we think all Asians are intelligent because we hear about or know many who get excellent grades in school. Maybe we think all people who wear glasses are smart because we see so many scholars and scientists who wear glasses. These assumptions are no more accurate than thinking all blonds are ditzy, or all tall people are self confident.
Do not berate yourself for your shortcomings. Instead of thinking "I'm so dumb!" work to change those thoughts to "Well, I messed that up, but I learned A,B, and C, and next time I will do better."
It does help to encourage yourself. Say (aloud, preferably) to yourself 'I am becoming more tolerant everyday' or ' I make fewer generalisations every day.'
For insight, try this experiment with a group: Select a random charactistic, such as eye color, height, left/right-handedness. (Try putting group characteristics into a hat and picking one blind.) Tell the group that there are chocolate chip cookies and milk for everyone EXCEPT (whatever characteristic is selected) and that the excepted group has to pass out the treats then leave the room. After the treats are passed out, ask everyone how they feel about getting cookies. Many will say they are glad they lacked the selected characteristic, and some will admit they feel superior because of this. Ask the excluded group how it felt to be denied on the basis of such random factors. Now switch it up a little, and tell the 'elite' group that they have to kneel down on the floor and tie the shoes of the 'excluded' group. How do the respective groups feel now?
Be prepared to learn things about yourself that may not please you! It is sometimes quite unsettling to learn that we are not as quite good (or as bad) as we think we are.