Harvested leaves should be used as soon as possible since they are perishable. Ensure there is a ready market before you harvest to prevent withering and loss of quality.
To reduce pests and diseases and to keep your soil healthy, rotate your crops. This is where you plant a crop from a different family. After kale (sukuma wiki), do not plant cabbages or spinach.
You will have healthier soil and a better crop. Legumes such as beans and peas are very good for your soil as they add nitrogen.