Well, it depends on your skills and what kind of portfolio your clients ask for. I can share some comments if you are a content strategist, UX engineer, or a technical communicator. You can prepare custom portfolio, such as content inventory or gap analysis, or a quick guide from whatever apps you use in routine.
(a) If you blog, prepare an ‘inventory’ of how you planned this blog, your branding process, and calendar (if any).
(b) If you manage any facebook or G+ brand page, prepare a quick guide to share how you post media, moderate the page, and see insights and analytics.
(c) You can prepare a story of how you plan and structure your resume (digital or paper), the conventions and styling, and the branding.
(d) Prepare a UX report for gap analysis and optimization scope, for any app you use (ToDo, Emails or for anything).
I always recommend NOT to use your portfolio from past employers. Create your own custom portfolio, and show the value.
Need more directions? I can discuss more on a call.