Kryptoroxx, your lawn is looking pretty good. Here are a few tips for growing a good looking lawn:
Fall is the best time to plant grass seed in the north. You need moisture and warmth for the seed to germinate. Late summer, say by mid August should be a good starting time.
Cut your existing lawn short and de-thatch, to take out dead grass that may be covering the soil in the thin areas. Give it a good raking overall. Grass seed needs to make good contact with soil to pick up moisture to allow the roots to develop when the seeds germinate. Be sure to remove excess grass.
Aerate and sow seed. I like the "Jonathan Green" brand called Black Beauty Ultra. It germinates in about a week with several light waterings per day.
As you water, the soil plugs from the aeration will break down and help to cover the seed, which will also help keep it moist to encourage germination. Black Beauty Ultra seed is good for sunny and shady lawns and sets deep roots to keep the lawn green with less watering. It has a nice deep green color as well.
Watering is the most tedious part. If you over seed, you have to commit to watering for up to a month for all the seed to germinate, especially with Kentucky blue seed as part of the mix."Keeping the seed wet" is the most important thing to remember when watering. If the seed drys out, it will die. All that water will help the new seed to develop sufficient roots. When you over seed, because the soil has not been tilled like a new lawn, it is harder for the seed to set roots. Watering, watering and more watering helps to overcome the rooting in undisturbed soil. The water will begin to penetrate deeper each time you water, which will help with rooting.
Add some starter fertilizer with sufficient nitrogen and phosphorus (Scott's starter fertilizer works well) to encourage root growth after about 3 weeks. When grass is about 4 inches, cut it down to about 2 1/2 inches. This leaves enough top growth for the lawn to develop but short enough for roots to expand out from the individual blades of grass.
Cutting grass short causes it to spread. Leaving it long encourages deeper root growth. To get the best of both, once the lawn is more dense, adjust the cutting height to 3 inches. Cut it again when it has no more then one inch of new growth (at 4" height).
If you have done all the above, you should be able to stand back and admire the fruits of your labor in 6 to 8 weeks!
Check out the pictures of my lawn over seeding job in the next block below this one, to see the results of my efforts.