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.