If you have low spots in your lawn, you can fill in these areas by adding a half a centimetre of soil or compost twice a year over several years. Avoid adding so much soil/compost when topdressing that you change the overall grade of your lot.
If required, spread up to a half a centimetre of soil or compost on top of your grass annually to increase soil depth. By adding this thin layer of soil or compost over your lawn, you add beneficial micro-organisms to help break down thatch.
You can topdress and reseed areas where your lawn is spotty each fall (also known as bald spots.) Remember grass will always be spotty under evergreens or in the shade. In these instances, remove the grass and establish plants which can tolerate these growing conditions.