Best way to decide what to change and how to change is based on customer feedback. A few ways to get customer feedback are observing, usage analysis, A/B testing and asking. Some of them such as A/B testing can be really costly if it's a major change.
What methods do you use to find what to change? Does brainstorming within the team and asking validating by asking potential users work? Which methods do you think are most useful and cost effective?
