http://www.winsornewton.com/uk/discover/tips-and-techniques/oil-colour/understanding-oil-painting-rules
From the Windsor & Newton web site:
Fat Over Lean
Each successive layer needs to be more flexible than the one underneath. This can be done by adding more medium to each successive layer, which makes each new layer more flexible than the previous one and stops the painting from cracking. Think of the rule as ‘Flexible over Non-Flexible.’
Winsor & Newton has a range of mediums to help create this flexibility within layers. One of the most commonly used mediums is Liquin Original and by using it, there is no need to keep on adding oil to your colour.

Thick Over Thin
When painting with heavy colour, it is best to apply thick layers over thin layers, this is because the thin layers dry quicker. For example if you like the impasto style of the Impressionists with their thick bold brush strokes then it is important to remember that these thick layers need to be upper most – thin layers on top of impasto layers are likely to crack.

Slow Drying Over Fast Drying
It is best to use fast drying colours continuously as under layers. If a fast drying layer is applied on top of a slow drying layer then your painting may crack. This is because the fast drying layers will have dried on top of layers that are still in the process of drying out, and as the slow drying layers dry, they will pull and twist those (fast drying) layers above causing them to crack.