Get the general look down. Hair, body, skin tone, clothing. Then shape of the face, general looks of eyes, nose, jaw, mouth. When your sim starts to look right, change the view between near and far, and look at your sim from different angles. Look at how the sims expressions affect the likeness. Try tweaking things more. Remember that as fantastic as the CAS is, there are some things your won't ever be able to do (crooked nose, the dimple in the right spot, the eyebrows just so). Looking at the sim from the full body view also lets you take in the overall look and the tiny details that bothered you don't seem so bad.
Also, sometimes a minor adjustment will make a big difference. Sometimes just moving the eyes a little closer together (or further apart) can give you that tingly feeling that you just created the digital version of someone.
If I really get into it, I'll spread the task over a few sessions, maybe saving different versions along the way and decide which I like best.
