Oil Painting Tip #1 Fat over Lean

The proportion of oil medium (not turps thinner) should be increased for each subsequent layer in an oil painting – known as painting 'fat over lean' – because the lower layers absorb oil from the layers on top of them. If the upper layers dry faster than the lower ones, they can crack.

Wikipedia Definition:
"Fat over lean refers to the principle in oil painting of applying paint with a higher oil to pigment ratio ('fat') over paint with a lower oil to pigment ratio ('lean') to ensure a stable paint film, since it is believed that the paint with the higher oil content remains more flexible.^[1]
Oil paint dries at different rates due to the differing siccative properties of the constituent pigment. However, everything else being equal, the higher the oil to pigment ratio, the longer the oil binder will take to oxidize, and the more flexible the paint film will be. Conversely, the lower the oil content, the faster the paint dries, and the more brittle it will be. Ignoring this practice, even in some alla prima painting, may result in a cracked and less durable paint film.^[2]"
http://en.wikipedia.org/wiki/Fat_over_lean