There's going to always be a reason for for the poorly growing and thinner areas in your lawn. However, this is not always obvious and may require you do some thinking to figure it out. Does the soil drain adequately in the thinning spot? Are insects an issue? Are there any diseases or nutritional deficiencies in your lawn? Too much shade? Too many dogs? Cutting the grass too low? Consider the answers to these questions before you take steps to recitfy the problem.
With the answers to these thoughts above, take a soil sample in the fall and perform and test to see what it's needing. Fertilization in the fall helps the lawn recover from the stress of the summer months. Generally, fertilizer (nitrogen, Phosphorus, Potassium) is applied in a ratio of 1-4-1. In some cases fertilizer applied very late in the season will keep the lawn green during the winter months and give the grass a boost in the early spring.
Patching thin spots is best done in the fall with the cooler weather. If more than about 50% of the lawn are is bare or infested with weed species than complete renovation may be necessary. The cooler fall temperatures will end the summer heat-induced dormancy, making it the perfect time of your for lawn renovation projects. Spring would be the second-best time to try to patch the areas requiring the most attention.
Small areas can be seeded by hand, but for large areas, a drop-style seeder is going to give the best result for overall consistency and appearance. A slit-seeder will combine the seeding with the actual aerating mentioned below.
Detaching and aerating is ideally done in the fall as the cooler weather aids in rapid recovery. The problem with disturbing the ground with these task in the spring is the fact that opportunistic weed seeds will try to germinate. Basically, detaching and aerating will reduce soil compacting and allow water and nutrients to reach the deeper roots.
It is important to also select the most appropriate grass for the location you are rehabilitating. For example, seed mixes that contain fescues are often an excellent selection for those shaded or partially-shaded areas in your yard.