Double digging is recommended on a new vegetable or flower bed and, on heavy soils, every three or four years thereafter. It can be hard work if you are not used to it, but you can introduce yourself to it gradually, there's no need to complete a bed in one session, half an hour a day will get the bed dug in a reasonable time without putting your back out.
The best time of year to double dig a new vegetable or flower bed on clay soils is in the Autumn (October/November) - or Winter (through to February) during a frost-free and relatively dry period, this will allow the winter frosts to help break down the soil ready for Spring planting.
On light, free-draining soil, Spring is prepared as the time to dig to avoid the Winter rains from washing out the nutrients.
When digging a vegetable or flower plot for the first time, you will never be sure whether the soil is natural or if it have been levelled or filled in with builder rubble, so the first time it is dug may well produce a large pile of stones, house bricks and goodness knows what - this may cause the soil level to drop and addition top soil may need to be added from elsewhere.
On thin topsoil over clay, you don't want to bring quantities of clay to the top. There are advantages in removing the clay topsoil from the site and bringing in good quality topsoil, however this will produce a wet area and good land drainage will need to be provided to ensure that water can drain from the lower depths.
On a square bed, the easiest 'plan of attack' is as follows (see diagram to the right):
This basic plan can be adapted to suit most shapes of vegetable/flower beds, just remember the process :-
All perennial weeds removed during the digging should be burnt. Any ash from burning the vegetation can be returned to the soil.