There's really no way to prevent the flu although vaccination can certainly help but it's not 100%. Because the flu is a virus there is no way to control the spread especially in busy and heavy populated areas. Preventative measures to try and ward off the flu or the worst of the symptoms is usually by helping to boost your immune system by ensuring a good balanced diet. Extra vitamins such as vitamin C can also help. If you also know that a certain location does have a flu outbreak you could always try and avoid that area for a few days, but again it's not full proof as viruses can survive outside host bodies from 12 to 48 hours in general, but people will continue to carry the virus outwith affected areas.
The main symptoms I tend to get is being extremely tired, very sore aching joints and muscles, a very heavy, lethargic feel in my body in general, headaches. These might also be accompanied by sore throat and other signs of a comon cold.