There have been some studies that have shown that people who were exposed to the Russian flu pandemic of 1889-90 were the most likely to die if they contracted the Spanish flu.
Examinations of the virus structure of the Russian flu and Spanish flu have shown that they had vast differences in the structure of their respective viruses. When individuals who had previously contracted the Russian flu contracted the Spanish flu, their immune response was simultaneously delayed and wrong. The individuals' immune systems started producing antibodies based on the Russian flu, but since the virus structure was so different, these antibodies had little to no effect on the Spanish flue. By the time the immune systems realized that the antibodies it was producing was not effective, it was too late.
A common theory related to the Spanish Flu and the generally younger people who succumbed to it is the idea of a "cytokine storm" which is to say that an overactive immune system could cause younger people to be more at risk than those with weaker immune systems.
Cytokines are small proteins that are used for intracelluar signalling, and in the case of immune response, they are released by immune cells to trigger inflammation.
In turn, inflammation is a signal to generate more immune cells to fight the issue at the site of the infection and throughout the body. The new immune cells thus release more cytokines and the process continues, but at some point, there is a regulation process which prevents the loop from spiraling out of control.
Nevertheless, while cytokines are critical to the immune response, they are also susceptible to becoming unregulated in certain circumstances and this disregulation can cause them to start doing damage.
Inflammation is helpful for fighting of disease, but it should be noted that some of the immune cells release toxins that are just as capable of killing normal cells as they are for killing invading cells. Maintaining the immune response for too long, and in too many organs can cause damage which might lead to organ failure and death.
It is unclear how this disregulation process would specifically work in the case of the flu, and there are other theories, such as the potential for those younger people to be in areas where birds infected with H1N1 were located. Obviously, the war would have concentrated younger people and also caused their movement over long distances to join the war.
We have had considerable advances that should blunt future influenza outbreaks.
There is significantly better observation and reporting of influenza outbreaks.
There are vaccination programs in place. They are not perfect, since you need to get vaccinated against specific strains, but it can be effective.
Medical personnel are also much better trained in general prevention of the transmission of infectious agents.
I'd say that simple reporting, communication, and good practices by professionals would be the most important methods for preventing a pandemic like the 1918 Spanish Flu (which probably started in the United States, BTW). There is some discussion of anti-viral drugs as well, but I don't think anyone is expecting to be counting on those.
Link from NIH that talks about much of the above in somewhat more detail: