Choose a fertile soil for the marigolds. If your soil is sandy or loamy, they'll still grow. Don't add additional fertilizer to the soil for marigolds.
Plant marigolds in a location with good drainage. If there's a lot of water run-off, they'll be too wet and prone to rotting, decreasing blooms.
Put the marigolds in a place where they'll get as much sun as possible. The sun will encourage them to bloom quickly and fully.
Plant marigolds only after all the frost is gone for the year, or they'll die before they get the chance to bloom.
Deadhead marigolds with your fingers. Just pinch the dead bloom and let it pop off in your hand. Regular deadheading lets new blooms develop and helps marigolds stay healthy.