A negative attitude can come in many forms. It can be hostile, depressing, or judgmental. If it is the latter, those of whom we have formed our judgments will often know how we feel about them even though we do not tell them. Our spouse, our children, our friends sense if we see them in a positive or negative light. Through our thoughts, we can bless those around us or doom them by casting negative expectations on them. Parents will sometimes inadvertently doom their children though they are making every effort on the exterior level to do the opposite. They develop an image of little Johnny as stubborn, and though they don't tell him this, Johnny picks up on their thoughts and fulfills their expectations. Oddly enough, it seems that children are more receptive to their parents' thoughts than to what their parents say. Perhaps it is because thoughts are sincere; words are sometimes not.
Direct your thoughts through prayer.
Spend some time in prayer each day blessing others. If you focus on the happiness and well-being of others, it will help you to replace your egoic vision of them (one that searches for weakness) with a spiritual vision of them (one that sees their essence).
Practice meta/loving kindness meditation.
Practicing loving kindness meditation helps you to extend compassion to others in the world. Like prayer, it draws you out of your ego allowing you to gain a spiritual vision of others, even those to whom you do not normally feel connected. And now there is scientific evidence that shows meditation and prayer actually change your brain.2
Recite a mantra when you are stuck on a negative image of someone. It might be something like "I free you of my negative thoughts" or "I wish you freedom from my limited vision of you." You can recite this in your head until the negative thought subsides.
Stop thinking of people in absolutes.
Remind yourself that the Universe is evolving and so is everyone in it. We are not the same people we were 10 years ago or even one year ago. We have changed. So have the people around us. Try to see them with fresh eyes each time you encounter them.
Above all, remind yourself everyday that your negative perceptions of others are delusions. God did not create faulty people. The faults you perceive in them are a consequence of them trying to experience the world through their ego rather than their Spirit. Liberate them of their ego by allowing them the chance to step out of it. Know them for their spiritual essence, and they will begin to know themselves the same way. Remember, it is not so much what you tell people that they believe, but what you think.