A few months back I completed a painting for a client in New York who commissioned a portrait reflecting the style of classical Hollywood.
I really loved working on it - it taught me to plan out my images beforehand, because I have always been someone who draws straight off in pen and ink without any kind of pencil guide, and I always felt like drawing in pencil first was somehow cheating (thanks, Aubrey Beardsley).
But actually, the longer the process of making something, the more room for discoveries to be made, and it's often where the best ideas regarding colour and composition have chance to step in.
Lately my process is: detailed light pencil sketch (focusing on areas of colour) > then watercolour > then add ink lines (sparingly).
If I draw in ink before I watercolour the tendency arises to stay too strictly within the lines, and the colours can't interact as much. There is still plenty of room for improvement - I always feel that I could paint much more loosely than I do - but it's all part of the learning process.