Providing much-needed lawn aeration for your grass entails dealing with thatch.
The build-up of lawn thatch makes it difficult for your lawn to breathe. Lawn aeration performed in spring or fall helps control lawn thatch. The process of lawn aeration (sometimes misspelled lawn "airation") can be as simple as poking holes in the soil throughout the lawn by walking over the lawn with spiked shoes, a practice preached by Jerry Baker, although only for superficial cases. For those in greater need of lawn aeration, Jerry Baker's suggestion won't be sufficient: you'll need to perform core aeration. You should also faithfully remove as much lawn thatch as you can in fall by raking deeply, rather than just skimming the autumn leaves off the top of the lawn.
Lawn aeration also breaks up compacted soil, allowing water and fertilizer to permeate into the root zone. Grassy areas submitted to constant foot traffic require lawn aeration more frequently.
Lawn aeration may be undertaken in the spring for warm-season grasses, as soon as the soil has thawed. I mentioned examples of warm-season grasses in a prior FAQ.
But for Northern lawns, some experts say the fall season is better suited to lawn aeration. If the soil is severely compacted, simple lawn aeration methods such as that mentioned above may not be sufficient. In such cases, go to a rental center and rent a piece of equipment especially for lawn aeration, called the "lawn aerator." This lawn aeration equipment will pull "cores," or plugs of soil out of the ground, letting air in. These plugs should be 2"-3" in depth. Such a plug should be pulled out of the lawn at about every 3". The plug-removal process is facilitated by watering the lawn the day before, but don't water to the point of muddying the soil.
Likewise, if your thatch problem is severe (say, 3/4" thick or more), rent a vertical mower from a rental center. Alternatively, hire a lawn service to do the job of lawn aeration for you. For more on dethatching, please consult my landscaping FAQ on raking leaves.