The best way to prevent the flu is to get a flu vaccination each year. The flu shot is an inactivated vaccine (containing killed virus) that is given with a needle. The flu shot is approved for use in people 6 months of age and older, including healthy people and people with chronic medical conditions (for instance: Diabetes, heart condition, asthma, COPD, etc).
The ideal time for vaccination is autumn, in anticipation of winter influenza outbreaks. Remember that you are still at risk of getting the flu for about two weeks after vaccination until your body has developed the antibodies that provide the protection against influenza infection.