Decide if your novel is large enough to handle multiple viewpoints. Short stories and smaller novels are not the best venue for changing viewpoints. Changing point of view works better in larger novels with more characters.
Find a central focus for your novel. Your characters should all have one central theme in common. This is important when you start switching point of views. Otherwise, you will leave the reader frustrated and lost.
Pick no more than two or three point of views for the characters. One character may speak in the first-person, while another character may be introduced using third-person. You need to stick with the point of view that you assign each character, however.
Signal to the reader that the point of view is changing. You can do this by separating the changes with chapter headings or paragraph headings. Another way is to simply add a bit of extra white space between paragraphs.
Use diary entries to signal a change in viewpoint. The diary is an efficient way to slip from third-person to first-person point of view. Likewise, you can insert quotes or clippings from fictional magazines or newspapers to move into another viewpoint.