Basil does not need many nutrients added to its soil. If the soil is high quality and fortified for gardening and houseplants, using a standard liquid plant food twice a month is sufficient. If the soil is a lower grade, adding small amounts of granular houseplant food at the initial planting helps basil thrive, as it slowly feeds the soil.
Basil loves sunlight. Whether planted inside or outside, moving it around to catch optimum sunlight will help it flourish. A light layer of mulch in the bottom of basil planted in the ground helps it grow. Regularly watering it from the base encourages growth.
Overwatering will cause the leaves to droop and fall off. Over fertilizing tends to make the leaves less flavorful and fragrant. Gently pinching leaves from the center encourages the herb to sprout more outer leaves.