Having done focus groups for several years now in different countries and industries, as I reflect back on them, I think of certain aspects that have worked well and thought I'd put those down in this post:
What about sample sizes? Focus groups are a form of qualitative research so clearly the number of people will be small (cost mainly). Thus it would be a leap of faith to assume that the entire population feels the same way. A good strategy is to conduct quantitative research based on insight/s generated at the focus group. In my experience that has been the primary benefit of focus groups - to generate insight based on drill down questioning of a small set of customers who are part of your target audience.
I hope this is useful.