Infections, like a cold or flu? The number one way to help prevent respiratory infections is WASHING YOUR HANDS. Always wash your hands before you do anything with your face: applying makeup, eating, handling contacts, or just touching your face for any other reason. This is the primary way you pick up respiratory bugs.
Infections in general: stay healthy. Eat well, exercise, get enough sleep.
If it seems you're getting an abnormal amount of infections, you might also see a doctor to find if there's some underlying cause. You might have allergies you don't suspect, for example; or diabetes can encourage poor healing in the skin; or something could be compromising your immune system, such as AIDS (okay, drastic, but it is one thing that increases your chance of sickness).