Thatch can a serious condition. Think of it as an "ingrown lawn" problem. Thatch begins when your grass clippings don't break down and continues when the roots and runners of grass cluster together to form a thick mat, rather than growing down into the soil. Lawns that grow into themselves instead of the soil don't take up nutrients or water in proper amounts, and are more prone to diseases. Fortunately, thatch is an easy problem to correct.
Rake up and collect your clippings, bag them and sent them to the city compost heap after mowing. Grass clippings will not break down in a thatched lawn, and may contribute to the thatch problem.
Test your soil's pH level. If your soil is too acidic, then amend it with lime to restore the balance. This promotes the health of beneficial microbes that help break down thatch.
Use a lawn dethatching product to break down the thatch. Most of these products contain beneficial microbes, enzymes or fungus that attach thatch to break down the grass clippings and other organic components.
Refrain from fertilizing your lawn. Fertilizer with too much nitrogen can kill the beneficial microorganisms that break down grass clippings. This can contribute to the thatch problem. Occasional fertilization is fine. However most lawns with thatch problems have been over fertilized in the past.