digitS'
Garden Master
This is from Wikipedia and one way — shovel first, spading fork to reach greater depth, second. Compost can be worked in with the fork.
This is about what I do but it varies. Removing 8-10 inches of soil is most important to me. The top soil depth doesn't really extend deeper than about 6 inches. A caliche layer usually exists below that which is difficult to break through. Also, the gravel content is high.
Once I have achieved that 8-10 inches, I am generally happy with it and compost liquids can percolate down from there. John Jeavon's Biointensive gardening approach suggests 24 inches.
Steve