A lot of good information in this thread. I'll add a few quick thoughts:
- Population growth (and projected population growth going forward) is a good way to get an idea on the basic direction of a metro area. It will quickly become apparent if the city is growing (such as Denver, Austin, Jacksonville, etc.) or shrinking (small towns, many cities in the midwest and rust belt). While you can find good cash flow in cities that aren't growing, it is unlikely your property will appreciate and it may even lose value or become harder to rent, which is a risk that should be considered. It is sometime hard to work with, but the US Census provides this data.
- Regarding submarkets, I think this type of stuff is best explored via relationships with realtors, other investors and local residents. I recently narrowed down my search from the entire NYC metro area to a specific area (Hudson County, NJ) and did so after extensive conversations with a number of people from all facets of the industry. A good place to start would be a Real Estate Investing Facebook group for an area you are considering.