Start as soon as possible, the more time you have for the plan to work, the greater advantage you'll have for growth.

For someone already in college I think the best route would be to just try to minimize the amount of student loans that you acquire so that you're not faced with a large amount of debt when you graduate.

A married couple filing jointly can deduct up to $6000 if they contribute that much and that is per student, per year.

There are some other things you have to take into account with risk. If you only have two or three years before college starts you really don't want to take the risk of the market going down and possibly not having as much as you started with.