A rototiller can break up the first few inches of soil in the area you intend to plant grass. This is ideal, as it will both aerate the soil and make it easier for juvenile grass plants to get a foothold on the dirt with their roots. Rototillers can be rented from most any hardware store and will go a long way toward getting your lawn started off on the right foot.
For a small area, such as repairing a patch of burned grass, manual tools may be all you need to break up the soil sufficiently. A pitchfork can loosen the first few inches of soil, and a metal rake carves deeper grooves into the loose soil. Sprinkle the seeds over the area by hand or using a spreader and cover the seeds with 1/4 inch of topsoil.
For areas with especially tough soil, or large areas, power seeding may be an option to consider. Most landscaping firms will offer one-time power seeding sessions. Power seeders are large devices that use compressed air to blast grass seed into the soil at the necessary depth for germination and growth.
Overwatering an area that has had new grass seed applied can wash away the seeds or drown seedlings. Keep pooling and puddling in the area to a minimum and keep the area well drained. Many homeowners mistakenly believe more water will help grass seeds sink into the soil, when in actuality this is the surest way to wash seeds away.