Watercolor on Arches CP 140lb, 9x12." Reference photo is Molly Peck of Julia Kay's Portrait Party. As you can see, 95% of this painting were done wet-into-wet. I was (and still am) very inspired by a fantastic book:
Ewa Karpinska takes the mystery out of the "difficult" medium and explains how to achieve the effects you want to see in your watercolor painting. She offers a systematic approach to wet-into-wet watercolor painting that makes sense. She breaks the "water cycle" (when water is applied to paper) down into seven stages: lens, mirror, shiny, semi-matt, cool matt, dry matt, and dry (only the five stages in between "lens" and "dry" are relevant to wet-into-wet painting). Using three different consistencies of paint (dilute, creamy, and pasty) and five color application methods (dropping in, tilting support, "skating", drawing, and "keeping a drop on a leash") she gives the reader 5 x 3 x 5 minus the incompatible combinations (like dry matt and tilting) = around 45 recipes for wet-into-wet effects.
Of course, the success of these recipes depend on variables like timing, amount of water, amount of pigment, and humidity. You still have to figure out the right moments for the right actions, and master the elusive medium by practice - but if you need to understand how it all works first, this book is great.
I was actually a little turned off by the cover when I was buying the book online, but as it was recommended to me by an artist I admire (Jean Haines), I went ahead and got it. And I am so glad I did! There are wonderful, dreamy paintings by Karpinska inside and several step-by-step tutorials as well.
Here is a brief demystification of wet-into-wet watercolor techniques at Handprint.com.