I guess I'm tutorial crazy! Here's yet another one - for how to draw lips. They are based off of my lips so I don't know if these same steps apply to a big masculine warriors sneer.
Step 1 - Start with a basic sketch of the mouth. Keep in mind that the most prominent line is the place where the two lips meet. At this point I've also gone ahead and colored the shapes in.:
Many people draw harsh outlines around the upper and lower lip so it ends up looking kind of cartoony. Of course if you want a cartoony look then that's ok, but either way you want to make sure your form is accurate. Another common mistake is that people draw the line through the center of the mouth very straight. Most lips don't look like that.
Step 2 - Start applying shadows. On my upper lip, I have a fairly defined line down the center. The line is illustrated below by placing a soft shadow on the right side of the lip.
Step 3 - With a soft-edged brush, throw some highlights in there. On many people, the bottom lip "pouts" out a bit, so its common for it to catch more light.
Step 4 - We're bringing out the shadows some more to increase contrast. Here I'm mainly focusing on deepening the crease between my lips.
Step 5 - With a fine brush, go in and make tiny vertical wrinkles in the lips. I used a yellowish-white tone for the highlights and a darker pink for the shadows.
Step 6 - We're done now. I just masked off the lips and upped the contrast a tiny bit to make things pop out a little more