Improve poor lawns with aeration and overseeding

Help the water reach the roots
Aerate compacted soil in the spring or fall to improve root development. Rent a power aerator for best results, or hire a professional. Note: these are large power tools that can be challenging to operate.
The soil should be moist, and making two or more passes gives better results. The aerator pulls up soil cores onto the lawn. Rake or mow to break up the cores. The soil cores will help decompose thatch layers in the lawn. If your soil is deeply compacted (more than 2 inches-dig a hole to find out) find a landscape professional who has equipment that penetrates 6 to 8 inches to aerate for you.

Overseed to thicken your lawn
Overseed, after raking or aerating to expose soil, with a perennial rye/fine fescue mix designed for Pacific Northwest conditions. Talk to a knowledgeable nursery-person. A light application of "starter" fertilizer can help the seeds grow quickly and crowd out weeds.

Dethatch when necessary
A 1/2-inch thatch layer can be beneficial, but much more than that can keep water, air and fertilizer from reaching the roots. Rent a power dethatcher and make several passes. This tool rakes up large quantities of grass to pull up the thatch. Rake up the thatch and compost it after dethatching. Then overseed to thicken the lawn and crowd out weeds. Note: these are large power tools that can be challenging to operate.

Choose the best time
April/May or September are the best times to aerate and overseed, or to amend the soil and replant a lawn.

You might want to start over
If your soil is very poor and compacted, it may be best to start again, fix the soil and replant.

Too many weeds!!
If very weedy, remove the sod with a rented sod stripper and start again.

Another option is to use a sheet mulch. Use clean corrugated cardboard over the grass (mowing before laying the cardboard helps level the area). Overlap cardboard sheets six inches and keep the cardboard and mulch four inches from the base of any existing plants. Spread four to six inches of compost, topsoil, grass clippings or a combination on top and leave undisturbed for 3-6 months. In approximately 3 to 6 months the grass is dead and a new planting area is created. Till and overseed in the fall or spring. Covering the area with ground tree waste, wood chips or saw dust also helps prevent weeds from germinating.

Test your soil and supply what's missing
Get a soil test to find out what's missing and spread the amendments (like lime) recommended in the test results.

How to reseed your lawn
Spread two inches of Grade A compost and till it in to a depth of 6-8 inches. Sandy or gravelly soils may need other amendments too--consult a certified landscaper for help with these soils. Rake the soil level, roll with a landscape roller, water to settle for a day, and rake again. Seed with an appropriate grass mix, and water daily if the weather is hot and dry until the lawn is well established. Consider hiring a qualified professional for this big job.