Think of an interesting person you've met recently. Consider what makes him interesting, and why you remember him. Write down as many physical and personality traits as you can about him. Now, write a character sketch. Make up the bits you don't know, filling in the blanks with your imagination. Create a well-rounded character by thinking through all aspects of him, including what he looks like, what he eats, his friends and family, his childhood, his quirks and important dates, such as birthday and wedding dates.
Imagine yourself, but change your appearance, the way you dress, talk, where you live and what you do for a living. Now, write a sketch of this character. There's a kernel of you in it, but it's not you anymore, so write the profile in the third person (using "he" or "she"). This method is useful for stories written in the first person. It gives you the freedom of playing out your better or worse self in a fictional setting.
Recall a character from classical fiction that really intrigues you and captures your imagination. Now, update her. Bring the character profile up to date for the 21st century. Alter her hair, clothes, how she speaks and where she lives if you need to. You could even write a story for this character based on an updated version of the original text: think of Amy Heckerling's 1995 film "Clueless" based on Jane Austen's novel "Emma" written in 1815.
Read news stories and feature articles to get ideas from real life. Create a composite character by mixing and matching traits from several people. For example, take the good looks and extravagant lifestyle of a tycoon and add a dash of psychosis. Write a profile of this person's behavior and what you think would happen during a regular day of his. Alternatively, take one very interesting real persona and make the appropriate changes to suit your story.
Review your family history. Look for interesting ancestors, or make them up. If your ancestry is from Scotland, imagine that one was a friend of William Wallace, the character "Braveheart." If your ancestors are from a rural African village, imagine what it would have been like to be the chief of the village, a farmer or a mother in time past -- or present. Chop and change character details as necessary. This is a fictional character, so you don't have to stick to facts.
Brainstorm -- just imagine characters. See what comes into your mind, and go with it. Perhaps you have a recurring dream or thoughts about a certain character that keep coming back to you. You're probably inventing these people for a reason, so explore why these characters appear to you and spur your interest, and then write about them. If you're a visual thinker, use techniques such as mindmaps and graphical orgnizers to help you to generate and organize ideas. Keep a journal or notebook with you so you have it on hand when inspiration strikes.