Cutting high and feeding properly will encourage your lawn to be vigorous. Vacant or thin areas fill in as a result. Some of the bare spots may need to be reseeded. To do this, rake the bare area up to remove weeds and loosen the soil. Seed with a good seed mixture, and feed with a fertilizer with a high middle number (eg. 4-12-10).
Even a light application of the cheaper over-the counter balanced fertilizers like 10-10-10 will do. Incidentally that’s the only time we recommend these fertilizers for lawns – during establishment. The best time to sow grass seed is mid August to mid September. Spring seeding often gives mixed results at best unless you are very faithful with a sprinkler.