Aerating Lawns
Why Aerate?
Aeration should be an essential part of your lawn maintenance routine as it allows deep root growth and helps the turf to become established. It also reduces the amount of thatch - the organic matter that builds up on the soil surface, made up of consisting of decomposing blades of grass, stolons and rhizomes. Whilst some thatch (about 1 cm) can be useful to reduce the amount of evaporation and protect the lawn from wear, too much will prevent water from reaching the soil. Removing thatch will also promote the growth of new grass growth, producing a healthy, vigorous sward.
When to Aerate
It is usually best to carry out aerating procedures in autumn, as the warm, moist conditions will encourage the grass to recover quickly. Avoid attempting these tasks in the summer, as they can make a lawn more vulnerable to drought.
The lawn should be mown to its usual height before starting work, then a low nitrogen fertilizer applied after you have finished. You should avoid using the lawn for a week or two to promote recovery.
Aerating Methods
There are four major methods of aerating soil and removing thatch: spiking, slitting, scarifying and coring (or hollow tining).
Spiking
Spiking may be carried out using a mechanical or hand spiker, or, for small areas, a garden fork. It allows air into the soil, thus encouraging root growth and reducing soil compaction. You should insert a hand spiker or fork straight into the lawn, and then lean it back slightly to let in more air. This will also create fissures in the soil, which will promote deep root growth. Mechanical spikers should be worked back and forth across the lawn as if mowing, using slightly overlapping runs.
Slitting
Slitting is carried out using a machine with knife-like blades that penetrate the soil to a depth of 8-10 cm (3-4 in). These blades cut through the thatch, allowing air into the soil and promoting a dense, healthy sward.
Scarifying
Scarifying removes thatch and allows air to enter the surface of the lawn. This is important as the soil organisms that naturally break down thatch need air to live. 
The technique may be done manually by vigorously raking the lawn with a spring-tined or wire rake, making sure that the tines of the rake reach well down into the soil surface.
Alternatively, a mechanical or powered scarifier may be hired and used instead. The machine should be worked back and forth across the lawn as if mowing, using slightly overlapping runs. The lawn should be scarified in two directions (one at 90° to the other) to remove the maximum amount of thatch.
Coring
Coring (or hollow tining) aerates the soil, removes thatch and relieves soil compaction, all in one step. A mechanical or hand tiner makes a series of 0.5-2 cm (1/4 - 3/4 in) wide holes across the lawn, about 10 cm (4 in) apart, removing a core of grass, thatch and soil. The holes should be filled with a sandy top-dressing to prevent them from closing. Coring usually takes longer to carry out than spiking and slitting as the soil is removed rather than simply pushed sideways.