By Justin Gerard
I have an upcoming illustration project that has warthogs in it. Before I begin, I am taking some time to familiarize myself a little better with these ugdorable creatures.
“I wake up in the morning, I look in the mirror, and I ask myself; just how did you get so good looking?”
I often do this before starting on a drawing that has animals or costumes I am unfamiliar with. (The drawings, not wondering how I got so good looking.) I like to do studies from life when I can, or photos when no models are available. 
As I draw from my reference, I try to commit everything I can to memory.  Most of this will happen automatically as you draw. Your brain beings to see certain things as correct and others as incorrect.  
Studies from photos
After you have worked from the photo it is amazing how much information your memory retains. Details you didn’t realize you were taking in come back to you as you draw.  
From Memory
The next drawings, done from memory with no reference around, will lack the sharpness and realism of the ones worked from the reference, but they will have a little more personality to them and I usually find them more interesting. 

 Once I feel like I have a decent understanding of my subject I start on the final drawings for my illustration.

Warthog Tracker #1

Warthog Tracker #2 

Later on, when I am into the final illustration I will bring out my reference again to fact-check, and make sure I haven’t put an extra leg in there somewhere.  My brain is good, but it has been known to trip over itself from time to time.

Most of this method of handling reference with illustration I blatantly stole from the Disney artists.  On some films, like the Lion King, they would have workshops where they would bring in an actual lion and do life drawings of it before they began work on the film.  It was a very good way of both keeping the personality in the character, while at the same time making sure it is anatomically coherent and believable.

Edit: (Thank you raphael for mentioning it!)
To see this process more clearly outlined and executed, check out Ian McCaig’s Visual Storytelling Tutorial from the Gnomon Workshop. It is a really excellent demonstration and has been very influential for me in how I approach using reference.