Bring a lawn back to life
There are some good reasons to overused your lawn, and learning how to do it really isn't difficult at all. Maybe you live in a place that has brutally hot and dry summers and you just couldn't keep up with the watering during July or August, now most of your lawn has died out. Over seeding can bring the lawn back to life without the need to rip everything out and start over. Or maybe your lawn just doesn't look as good as it once did. Grass gets "tired" after about 5 or 6 years and doesn't replenish itself the way it once did. If your lawn hasn't had any new seed added in 5 years or so you might be due for an over seeding.
The best time to over seed your lawn is in September. Weeds are not as abundant in the fall so the new grass will have less to compete with. The summer heat is behind us, yet the soil is still warm to help germination. And seeding in the fall gives the new grass time to take root before the winter freeze hits. If you missed your opportunity in the fall, don't worry Spring works too.
Now that you've decided to over seed and you know when you are going to do it, there are some steps to take to prepare.
The first thing you'll want to do is mow your grass as short as you can, and bag the clippings while you're doing it. The new seed will only germinate if it is contact with soil. Sitting on top of grass clippings just turns the seed into dinner for the neighborhood birds. Another thing to do to make sure the seed gets to the soil is to remove any excess thatch that has accumulated over time. You can do this with a grass rake, the best ones are metal and scrape the top of the soil while removing the thatch. During this process you should also remove as many weeds as possible.
Another good thing to do is aerate the lawn if you have the time and means. Aeration allows oxygen to get down into the soil and will help the new seed grow faster. It also helps with the de-thatching if you didn't manage to get all the thatch up. Aeration is something you should do regularly, but before over seeding it's especially helpful.
The next step is to top-dress the yard. Top-dressing with a compost/topsoil blend can help by introducing organic matter into the soil which will help germination. You should top-dress about 1/4" to 1/2". After top-dressing, water thoroughly, up to an inch per day. This makes the soil moist and ready for the seed to germinate.
Now that you've done all the prep work possible it's finally time to do the over seeding. A great tool to use for this is a Slit-seeder. This tool actually makes little slits in the soil and drops the seed into them. You'll get much more germination with a slit - than with one of the alternatives. Of course if you can't get access to a Slit-seeder you are not completely out of luck. There are other options such as a broadcast spreader or a push drop spreader. If you did do all the prep work and did a good aeration job the type of tool you use is less important and you just want to make sure you get an even layer of seed out.
After seeding, fertilize with a good grass starter and then water. After that first water you should water at least twice a day for the first couple of weeks. Do not allow the new seed to dry out, it needs to remain moist for the best germination. After full germination you can water less often but still make sure it gets soaked.
After finishing the process, it's best to keep any foot traffic off the newly seeded lawn. A couple of stakes and string can help remind kids and even ourselves to stay off for a while. You need to resist the urge to mow for 2 -3 weeks.
If you follow these steps and do the over seeding in September, by next April and May your lawn will be looking great. Just in time for BBQ season.