Currently - I have a large list of clients who prefer to use NDA's, however I recommend that you ask them if you'd be able to show their work on your portfolio. A lot of clients are open-minded to this.

I've found that most of them prefer that you don't show it, until it goes live. This can also be a good counter point if they won't allow you to show it right away. Most often you'll need to list out the entire team alongside your contribution. Art Director, Creative Director, Designers, Researchers, Developers, etc. My best suggestion would be to think about these projects in your portfolio as case studies - That might help you with your format as well.

You may also find that you can share work in-person but unable to publish it. This would include job interviews, pitches, etc. But again, you'll need to get permission from the client for those as well.

If however, a client is unwilling to give you the permission (Which I'd get in writing) then unfortunately there is nothing you'll be able to do, to show that particular project. When working for a client your work basically becomes their property and all rights or attribution to it becomes theirs unless outlined in an earlier document upon starting the job.

Hope that helps!