Giving characters a distinguishing feature or mannerism increases your story’s imagery and provides a handy mechanism to remind readers what they look like. This can be something like their hair color or style; other distinguishing physical features such as eyes or nose; or certain gestures.
The more characters your story has, the more important it is to give them each some sort of “tag” so readers can keep them straight. With the possible exception of red herrings in mysteries, everyone in a story needs to serve a purpose and move the plot along. If they don’t, zap ’em, and if they do, make them memorable.
When you have a huge cast of characters, remind readers who the minor ones are from time to time so they can keep them straight. Placing them in a scene that fits their role sometimes will suffice. Having a dramatis personae is also highly recommended. This is a list of the people in your story and who they are. In highly populated novels these are greatly appreciated by readers, especially those without steel-trap memories or who may take a little longer to finish a book.
If your book is published in both print and electronic form, make sure the character list is included in the e-book’s table of contents so readers can refer back to it more easily.
If your book is loaded with characters, do your readers a favor by providing a dramatis personae, a fancy Latin term for a list of who and what the players are which you provide in the beginning of the story; a cast of characters, if you will.
Not all readers have a steel-trap memory that can keep track of too many people. This is another thing that can throw a reader out of a story, wondering or trying to remember who someone is. For ebooks, make sure this list is included in your table of contents so readers can get back to it easily for reference.