Here is another 'anatomy of an illustration' piece.
I have a few different techniques that I use.
Here's one. This piece was done for LifeWay Christian Resources a while back. It was for a children's Sunday School curriculum I believe.
First, I did a sketch to present to the client.
After approval, I took the drawing into photoshop and adjusted the hue, saturation, and lightness (image/adjustments/Hue-Saturation) until I had a light blue looking sketch. Then I printed it.
I drew over top of the blue lines with a brush pen and fine-tipped fancy shmancy art marker. I buy the heavier, brighter stock paper, so that I can do this technique without the ink bleeding. Then, I scanned the black line drawing back into photoshop and further adjusted any little dots or mistakes I may have made in the drawing step. (in the case of this illustration, I didn't have any errors, as far as I can tell)
Then I exported the black line drawing to Illustrator, did a 'live trace' to convert the line drawing to vector, and then colored it, and added shadow shapes etc.