I would suggest working with a medicare supplement (medigap consultant) plan broker. There are many plans available in each state. Since plans are standardized, you basically compare costs and financial strength of the company. Working with a broker allow them to shop your medicare supplement plan each year.
Be wary of well known plans that advertise a lot. You will be paying for those ads by way of higher premiums. Since each medicare supplement plan F is the same no matter what company; so you can choose the lowest premium.
Don't go direct with a company either. The premium will be the same as with a broker, but a good broker will notify you when there is a better plan available when rates change. A direct agent or captive agent will not be able to make that call to you because they won't have any other options. I work with Seniors every day that don't realize how the standardization of the plans helps them. They are paying high premiums for plans that can be purchased for much less.
When you look at rate increases, the percentages can be misleading. If AAYZ is charging $180 and goes up 5%, and XYZ is charging $90/month for the exact same plan and increase by 15%, tell me who is helping out the senior? It's the exact same plan with the same coverages. With AAYZ, you do at least get a magazine... Tell me who is making a fortune. With that said, many seniors still cling to the big names, who in turn hit them with high premiums. I work with those on the fixed incomes. The relief I hear when we are able to lower the premium and keep the same benefits lets me know I am making a difference.