Ventilation can't eliminate all the heat that builds up in your attic, but it can minimize the effect on your home. In order for attic ventilation to be effective, you must have equal intake and exhaust vents. Therefore, for every soffit vent installed you will have to install a roof vent, or install a ridge vent. The idea is that the soffit vents allow cool fresh air into your attic while the roof vents exhaust the hot humid air.
A general guideline for determining the number of vents you need is at least one square foot of intake vents (soffit) and one square foot of exhaust vents (roof vent) for every 150 square feet of ceiling. You will want to consult your local building codes to be sure that you are compliant. For more information on ridge vents, see our tutorial "Installing a Ridge Vent" - and remember, a cool attic is a happy attic!