What you’ll need
Correct the Grade
Get rid of low spots and uneven grass surfaces by raising the grade of the low spots. Adjust the grade of your lawn by adding soil a little at a time. During the growing season, add from 1/2 to 1 inch of soil at a time and rake to raise the grass blades above the new soil. The grass will survive and raise itself to the new soil level in a couple of weeks. Then add more soil until the grade is where you want it.
Aerate
The top 2 inches of soil can become compacted from foot traffic. Compaction causes the grass to become weak and allows weeds a foothold. Break up the compaction by renting an aerator. This power tool is driven across the lawn, where it removes plugs of soil about 3/4 inch in diameter and 3 inches deep. Make repeated passes until the holes are no more than 3 inches apart. The holes through the compacted layer allow water and air to pass, and encourage deeper root growth.
Fix the Drainage
If the drainage problems are only small areas, correcting the grade to raise them so the water can run off will correct the problem. If they are larger areas, however, it will be necessary to increase the surface area of the lawn to absorb water faster. One simple way to do this is to cut a series of parallel trenches with a chain saw. Make the trenches as deep as the chain will reach and about 6 inches apart. Cover the low area with them. Use an old chain—cutting the soil will ruin it.
Fill the trenches with coarse graded sand, the kind used to fill sand boxes. Beach sand will also work, but wash the salt from it first. Fill the trenches to the surface and mound each one slightly to keep loose soil from washing onto it and sealing its surface. The grass will soon grow over the sand, hiding the trenches. The trenches greatly increase the surface area of the soil that can absorb water, improving the drainage.
Add Organic Matter
Organic matter opens and loosens soil, making it a better environment for plant roots. Its effect is accumulative—each time you add some, the soil improves more. Spread an inch of organic compost across the lawn surface and rake the lawn to raise the grass blades above the amendment. The amendment will fall in the aeration holes, moving it into the soil. In addition, earthworms and insects will feed on the amendment, moving its nutrients deep into the soil. You can add more every month or so during the growing season to continually improve your soil.