One of the best times to repair a sparse weak lawn is in the fall. Cooler temperatures have arrived and no doubt increases of natural water in the form of rain.
There are many reasons why grass fails and you need to decide just what your particular problem is before you can fix it. Study your lawn to pin point your particular problem before you waste time using wrong techniques in trying to remedy it.
Many lawns over time become victims of compacted soil. Heavy foot traffic and mower weight can pack the soil, especially if high in clay, creating a top surface that is difficult for water and air to penetrate. Even though you are watering regularly, the water is not reaching down into the grass roots. Test your soil by pushing a sharp tool into the ground. If it is very difficult to push your soil is most likely compacted. Rent a lawn aerator which will poke small holes into the turf. In future you should aerate your lawn every year or at least every two years.
Your lawn problems might just be you, the gardener. Regular lawn maintenance is necessary for lush grass. Assess how you are doing your job. Are you overwatering, over or under applying a lawn fertilizer, or perhaps cutting your grass too short. Improper lawn care will lead to stressed grass which will then be more prone to turf problems.
If your yard is now mature it could be that your trees are over shading your lawn. Many turf types rely on a good dose of sun to stay healthy. Rather than prune those lovely trees I suggest you over seed your lawn with a seed variety which is more shade tolerant. Lawn seeding will slowly fill in where the other grass dies out. A best of both worlds solution.
If you are overburdened with weeds, disease or lawn pests you might have to break down and use a product for your specific problem. Choose the most environmental sensitive product you can find. In the case of weeds rather than using a herbicide dig them out, roots and all by hand. Applying a pesticide or herbicide should only be used in extreme cases and only once to get the problem under control. The key to pest and weed prevention is a strong healthy lawn in the first place. Over seeding with a variety more resistant to pests and diseases should help.
Read up on proper long care and develop a lawn maintenance schedule. Your results will be a greater lawn with less work and effort on your part in the long run.