This great video by Winsor & Newton succinctly sums up the three basic rules of successful oil painting.
They show there are three basic, tried and tested rules that will ensure a great finish to your work:
Fat Over Lean
Think of the rule as ‘Flexible over Non-Flexible.’ Keeping each subsequent layer more flexible than the one underneath.
This can be acheived by adding more medium to each successive layer, so making each new layer more flexible than the previous one to stop the painting from cracking.
There is a wide range of mediums available to help create this flexibility within layers, Winsor & Newton produces one of the most commonly used, their Liquin Original and by using this it stops the need to keep on adding oil to your colours.
Thick Over Thin
Again to avoid layers cracking it is best to apply thick layers over thin layers, this is because the thin layers dry quicker, especially important to avoid when painting with heavy colour.
For example if painting in an impasto style then remember that these thicker layers need to be upper most. Done the other way around thin layers on top of impasto layers are most likely to crack.
Slow Drying Over Fast Drying
If a fast drying layer (due to thickness, colour, oil or medium used) is applied on top of a slower drying layer then your painting may crack, because of this it is best to use fast drying colours continuously as under layers.
If this rule isn't followed then faster drying layers on top will have dried quicker than the lower layers that are still in the process of drying out and as the slow drying layers dry, they will pull and twist the faster drying layers above causing cracking.