Ultimately, the best thing that could be done to prevent a Katrina-like disaster in New Orleans is to not build structures in areas below sea level near plentiful standing water in a hurricane prone area. Much of New Orleans lies below sea level, and a complicated series of levees holds water back from the Mississippi River and Lake Pontchartrain, and of course the Gulf of Mexico is not very far away. There are also a series of drainage basins operated by sump pumps to remove rainwater from the city. Obviously it is too late for not developing low-lying areas, so reinforcing the levees and pumping stations is definitely the best way to prevent the kind of widespread flooding that devastated the city during Katrina.
Another way to prevent disaster is to have a better plan of evacuation in place for the New Orleans area. Too many people either did not or could not leave their homes and evacuate the area who definitely should have. Putting a more comprehensive evacuation plan in place including the use of buses and public transportation for those who do not own cars would help mitigate the loss of life seen during and after Hurricane Katrina.