A few weeds are inevitable in most lawns. Pull weeds, roots and all, by hand or with a handheld weeder (three good types are: fishtail, leverage or Japanese) or hoe (either a garden or stirrup hoe will work). If the weeds are out of control, use an herbicide such as glyphosate for direct application to weeds or apply a weed-and-feed product to the entire lawn; follow the manufacturer's recommended application instructions. Be aware that these products are effective only when the weeds are properly identified and the product is applied at the right time of year.
Thatch, or the matted accumulation of organic debris between grass blades and roots, can cause dead patches in turf and open spaces for weeds to grow. Inspect the lawn for a one-inch layer of thatch. Use a thatching rake or power dethatcher to remove thatch. Afterwards, the lawn will look terrible, but it'll recover in about three to four weeks.
Inspect turf for compaction problems. Dig a square-foot section of the lawn and examine the roots. If the roots don't extend deeper than 2 inches, then the lawn needs to be aerated. Water the lawn one to two days prior to aeration. Run a rented core aerator over the lawn once. Apply compost or sand over the aerated lawn.
In early spring or early fall, overseed a lawn with cool-season grass seed such as rye, fescue or Kentucky bluegrass, and overseed a warm-season lawn with Bermuda or Zoysia grass in early summer. For an even application of seed over the lawn, use a broadcast seed spreader. Water the seed to help it establish.
A healthy lawn is denser and better able to resist weeds than a neglected one. Don't scalp the lawn; remove only about one-third off the top of the grass. Set the height of the mower deck so that the grass is about 2-1/2 to 3 inches tall. If the grass is taller, it helps to shade out weed seeds and prevent them from germinating. Alternate your mowing pattern by mowing at a 45- or 90-degree angle from your last mowing direction. This helps prevent soil compaction and also helps grass grow upright. Keep mower blades sharp.