Have you ever read a scene in a book and realized you had no idea what that character looked like? I really appreciate it when the author provides that information. However, I’ve heard some say that they think the reader would prefer to imagine him or her as they like. Personally, I think that is the author’s right and responsibility. If I want to invent a character, I’ll write my own story.
There’s also the matter that some descriptions need to be repeated as reader reminders. This is especially true of minor characters who may have been described several pages or chapters previously. It never hurts to drop a hint every now and then. Another great identifier is a gesture or some other habit that is linked to a given character. For example, maybe she tosses her hair or he plays with his mustache.
This same advice goes for certain settings and locations in your story. If you do a good job the first time, you can minimize them later. Imagery makes a story come alive. If you want your reader to visualize the story as you did, you need to provide this information.