Assess your current lawn. In most cases, you can revive a lawn by removing weeds and taking preventive measures. However, if weeds make up more than half your lawn, consider starting fresh by reseeding or rolling out sod.
Identify whether the weeds in your yard are grassy or broadleaf. As their names imply, grassy weeds have blades (think crabgrass, foxtail and sedges), while broadleaf weeds have veined leaves (think dandelion, chickweed and clover).
Purchase a mild post-emergent herbicide made for your type of weeds. Make sure the bottle indicates that the product kills weeds, but not grass (Roundup, for instance, would be too strong). Apply the herbicide directly to weeds. For an organic alternative, you may choose to remove weeds the old-fashioned way---with your hands, garden gloves and cutting tools.
Apply a pre-emergent herbicide each spring. This will prevent new weeds from cropping up when the weather warms, while still allowing grass to sprout.
Mow often, but cut blades to no less than 2-½ or 3 inches tall each time. Mowing is about achieving a balance. Cut too low, and blades will grow rapidly to reach sunlight, instead of spreading out and strengthening roots. This weakens grass and invites weeds. But mow too infrequently, and your tall lawn will prevent proper aeration, roots will weaken and weeds will also overwhelm. It might seem like a lot of effort at first, but the payoff is healthier grass that is better able to box out weeds.
Water deeply but infrequently to encourage strong root growth. Watering too little, too often can weaken roots, allowing weeds to spread. Water enough to moisten soil about 6 inches deep, but not enough to flood the lawn (which can lead to diseased roots). Try a prolonged watering every few days at first, and adjust based on your lawn's reaction.
Fertilize with a product made for your type of lawn at least twice per year, in the spring and fall. For a lawn that needs extra help, you can fertilize every 4 to 6 weeks fall through spring. Avoid fertilizing in summer, though, as it will only encourage weed growth.