One of the keys when applying a completely new bed of topsoil is to apply it at the proper depth. It's a mistake to spread topsoil in a layer that is only a few inches thick. Instead, you want to lay down a nice thick layer of topsoil that provides a good root environment for your plants or grass. Ideally, you want to add between four and six inches of new topsoil on top of your existing ground.
When a lawn starts to die off in certain places but the rest of it remains viable, you can over-seed a section of the lawn to get your grass to come back without starting from scratch. Once a thick layer of seed has been applied to the area, apply a light layer of topsoil over the area that you have seeded. The key here is to roll the topsoil firmly into the ground so that the seeds are all embedded in the topsoil, or between it and the existing soil. This provides the perfect medium for new roots to start growing in.
When you need to put topsoil down, it is important that you do it when the conditions are right. This is especially true when you put down a thicker layer of topsoil, such as in a garden or when replacing your lawn with all new sod. Anytime that the ground isn't full of frost is appropriate for applying topsoil, but you need to make sure that the weather is dry and that it is not windy out. Trying to apply topsoil on a windy day can result in your topsoil blowing everywhere.
When you apply topsoil to a garden or other new soil preparation, you need to make sure the topsoil and the subsoil are able to mix together. Otherwise, you end up with two distinct layers of soil that moisture and nutrients don't travel between well, especially if the lower layer is compacted. Use a rototiller or a hoe to break up the soil throughout the garden where you want to spread the topsoil. Ideally, you should break up at least the top four inches of the soil before you spread topsoil.