Note that if you're sharing work that isn't public, your employer might have policies forbidding sharing anything. You asked how, not whether, so this answer assumes you have the permission.
You can list the details of your projects, as suggested in another answer, but that tells only part of the picture. That tells an employer that you've used such-and-such technologies to do such-and-such tasks, but it doesn't show whether you're any good at it. A portfolio is a supplement to a resume where you show, not tell.
What you put in that portfolio will depend on the nature of the work:
If you developed the visual design of a site you can use screenshots.
If you did interaction design, use either videos or UI mock-ups showing flows. I would go ahead and include Balsamiq mock-ups (or whatever your tool of choice is) here, but assume that many people won't be able to use them. This is a "for those who can" enhancement.
If you wrote code or did back-end work like optimizing a database schema, that's harder to show. If it doesn't violate the policies (or NDA) of the company for whom you did the work you can show code samples, same as for people applying for programming jobs, but you're going to be more limited here.
In addition to the above, always include an explanation of why you did what you did. What were the important design requirements? What processes were you trying to optimize? What platforms did this have to support? Who were your users?
