How do you divide the USA into 10 even sales territories by population?

Answer 1

Zip code. The population is pretty evenly distributed by the first digit.
Of course, if you are selling snow shovels or swimming pools you might need different distributions.

Answer 2

Try this site for free software to download a sales territory map: http://www.smartdraw.com/downloads/index.htm?WT.svl=link07
Then use information from this location for populations and tie the two together : http://members.aol.com/bowermanb/population.html

If you want to buy a map already set up, there are many companies that provide these services. this one offers a free trial:

www.DemographicsNow.com

Hope these help!