Based on actual research of your user groups, or a brainstorm, identify the key charactersthat you are going to design for.
Give the characters names and visually represent how they look and dress, their aspirations, behaviour, lifestyle and any challenging peculiarities.
It is important to create profiles of extreme users as well as typical ones. It can also be useful to write 'stories' about a typical day in their life.
Display the profiles prominently. They will help you to stay on course and stop you designing for yourself. At decision points, ask yourself, 'What would Mary or John think of this?'
You could...
...make composite profiles by merging the characteristics of real users you have met.
Resources
Photographs and images cut from magazines, a sheet of flipchart paper or foamboard