Remember that the only safe assumption to make is that your first assumptions might be false. This keeps your head level and your mind open.
Recognize the assumptions you make in everyday life and think about whether they're useful. Throw out anything that doesn't help you live and work more efficiently.
Use riddles and other divergent thinking exercises to expand your ability to think critically without making false assumptions. These are some of the best ways to break bad mental habits.
Respect other people's ideas. Even though you might disagree with someone else on a hot topic, recognize that her opinion isn't worthless simply because it's not the same as your own.
Be as objective as you can in making evaluative decisions. Whether you're at work, at home or with your friends, be as logical and fair as possible in your judgments.
Resist the urge to accept stereotypes. Even though they might be accurate, stereotypes are reductive and can cloud your judgment.
Base your judgments only on the most apparent facts. This is the most logical way of doing things, and it'll stop you from make false assumptions altogether.