What are their personalities like? You can work backwards from there, try and find out how society might treat them for them to have become that way.
I'm a pretty visually-minded person, so that aspect comes fairly easy to me. But when I do get hung up, I start with the broad strokes: *Is he younger or older? What age group makes sense for the personality and profession I've given him? *what's his race/ethnicity? if he already has a name, what fits the name I've given him? Italian, Korean, Cuban, etc.? Once you answer those two questions, you can start getting a little more specific: *Build. Is she an athlete? Does she meticulously restrict her calories? Or is she a woman of ample physicality? *Hair and eye color. The race question influences this, as certain traits are common to certain ethnic groups. For most characters, that's enough of a description, as the reader can fill in plenty of blanks themselves. However, if you feel like you want to get an even clearer idea, you could do what I do and pick out actors who you feel could play the part, and imagine them when you're writing. Should a particular scene require mentioning a particular detail, like a love interest drinking in their features or an antagonist memorizing their enemy's face, you can list features belonging to Jennifer Lopez or George Clooney or whoever.
I can never picture my characters, but then, I can't picture faces in my mind at all. Thus why I fail to recognize people I know very well all the time... But still, I do google image searches and pinterest and I found pictures for all my characters that help me describe them.
I know this is an old thread, but I came across it searching for something else and I thought, what the heck, I'll add my two cents ... I came across this recently. I had a basic description in my mind of what my characters look like. But sometimes, I can only see hair, and eye color and I'd like to see an actual face, to help with my own visualizations. I don't intend on describing their features in detail but when I'm typing a story and I can see the characters in full detail it helps me move the story along easier. This is what I did: I googled actors or actresses that fit a vague description, and I go thru the list (usually in IMDB.com) and find obscure actors. I didn't want famous actors because I felt it would taint my character development, instead I'd pick actors that I'm totally unfamiliar with. Maybe that's strange, but that's what I did, lol! I did consider using Sims, which I thought was funny to see how many people did that, but I felt it too cartoonish, I wanted to visualize a real person. Anyways, that's my two cents.
I usually see a photo online and have a mental image stored. As all humans, my memory doesn't remember exact details so my mind alters how they look slightly, enough that I know what they look like but they aren't the same person as who I based them after. Which is very helpful because it makes them unique.