SurvivalBlog writes:

According to statistics published some 20 years ago by Dr.Ravi Batra, the safest businesses and industries during the worst years of the Great Depression (1929-1933) were:

Repair shops
Educational services (A lot of young men that couldn't find work borrowed money to go to trade schools and college.)
Healthcare services
Bicycle shops
Bus transportation
Gasoline service stations
Second hand stores
Legal services
Drug or proprietary stores

To bring Batra's list up to date, I would speculatively add a few more sectors and business that are likely to do well in the next depression:
Home security and locksmithing (since a higher crime rate is inevitable in bad economic times.)
Entertainment and diversions, such as DVD sales and rentals. People will undoubtedly want to escape their troubles!
Truck farming and large scale vegetable gardening (since just 2% of the population now feeds the other 98%--whereas back in the 1930s the US was still a predominantly agrarian society) Export consumer goods. (Starting in late 2009 or early 2010, the US Dollar is likely to resume its slide versus most other currencies)