Apply a fertilizer rich in nitrogen to your lawn in the spring or fall when there is less chance of higher temperatures and more moisture. Use a garden hose with a hose end sprayer filled with the fertilizer. Follow the instructions on the specific fertilizer you purchase.
Mix 1/2 lb. of coffee grounds with 5 gallons of water in a bucket if you don't wish to use chemicals to raise the nitrogen level in your yard. Apply the mixture to your lawn in the fall using a sprinkling can. Be aware that the coffee grounds can raise the soil's acidity, so only use this method if your lawn has a higher pH level.
Plant soy beans, alfalfa and clover in the fall. Till the plants into the soil as soon as you can in the spring and test the nitrogen level again. These plants are high in nitrogen and can provide a completely organic method for raising the nitrogen level. The drawback, of course, is that you must re-seed or re-sod your yard once you've tilled the soil.