It takes a bit of planning to forestall something, meaning stop it from happening. To forestall the effects of aging, exercise and take care of your health all your life.
You can break the word forestall into parts to figure out its meaning. The prefix fore is one you've seen in words like forewarn, which means "to warn in advance." And you probably know that stall means "delay." So to forestall is to stall in advance, or put another way, to try to prevent or put off something you don't want to happen.