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.

For example:

(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.

If you don't want to work for free and are starting from scratch, then I recommend doing the following

- make up sites. Create websites that come from your imagination. These can be riffs off of existing sites if you like but make sure you don't just copy something and change the main pic. They should be entirely of your creation.

- redo an existing site and show a before and after. Include a case study to show how and why you made the changes.

The point of a portfolio is to showcase your work. Whenever I review a candidate, the first thing I do is check that portfolio to see if their level of design is what I'm looking for. Doing the above will show the viewer that you are experienced and competent, even if you don't have a ton of companies under your belt.

If you are already engaging with customers but are losing them because of a weak or non existent portfolio, then spend some time looking over their site before you meet. Put together a plan of what you would do to improve it and walk that customer through this plan. This will help establish you as someone who is not only knowledgeable, but actually vested in your customer's success. Doing this prep work will go a long way towards landing the client. Especially since few people take the time to do so.

Do spec work for large companies, but, don't just redesign something for fun.

Find a company that has a problem with something that relates to your field (for example, a UX Consultant, like me) and then solve that problem with your redesign.

Explain what the problem was, how you solved it, and the value you created for the business by implementing your solution.

No one cares about your design, they care about the value you're creating for their business. Focus on that.

Hit a cause. Something current and of value and that strikes a chord with you personally. Create a piece of content that reflects not only your skill but your passion for the cause. This will obviously not come across that you are a pro bono machine as it was a personal cause you were committed to while at the same time creating a piece of work that accurately portrays what you can deliver for a client.