When UX professionals get together to redesign a service or product a common practice they will employ will be the use of personas. Personas are representations of different kinds of people / users of the thing you are redesigning. They are the archetypal kinds of people using the thing you are looking to improve. They are the novices, the savvy experts, the casual users, the die-hard fans, the skeptics, etc. You give them names, you describe what their current needs are, how they use the product today, as well as their expectations for the future.
While you can't please all the people all the time, this exercise forces you to account for a variety of users and to remember that you are designing for a variety of needs and expectations and not just your own. It pushes you to step outside of yourself. Personas have the added benefit of putting a face to a series of requirements. It helps everyone involved keep organized by having a name to go with a list of features & requirements.
So where to begin?
You can start a few ways. If you have the luxury of time & access you can interview stakeholders in the project from a variety of levels. From the hands-off to the hands-on users, from the executives down, you will gain insights from people all along the way. If you don't have that sort of time, brainstorm the kinds of users you are designing for. What will the executive level user want or do with your application? What about the person who is doing the day-to-day maintenance of the app? Create lists of the kinds of users, how they are doing things today, what they want in a future design, what their roll will be, their level of technical sophistication, etc. From here you can begin to design for a variety of consumers.
A word of caution.
Personas aren't real people. It seems obvious but it is worth mentioning. Personas are a great way to get organized and remember to account for a variety of users but they aren't the same as actually testing your design with real people. You should still validate your ideas with a variety of people and see what's working and what isn't. A persona will guide you to a solution but you still need to confirm, with real people, if that solution works.