The best place is in the Header so that is accessible from every page on your site. If you can not alter your header, then the Home page and any Registration pages should be used. Make it noticeable and encourage your clients to use it often to manage their account. You can name the link anything you'd like to. View some various Portal Login names and labels on the Link to Your Parent Portal page (look under the gold box and click open the view) to get ideas.