A layer of soil is an important part of planting grass seed because it protects the seed, keeps it in place and encourages faster germination. Too much soil isn't desired for the sprouting grass to have to fight through. Cover it with just enough soil to get the process going.
Rake the seed lightly after sowing it. Running the back of a metal rake over it works just as well. This will mix the seed with the soil, covering it with a layer that is 1/8- to 1/4-inch thick.
Cover grass seed with a think layer of straw if there is a concern with the seed staying in place. This is the case on slopes, for example. Pick up the straw once the grass starts sprouting.