Option A (Decent): Making a good-looking portfolio of work you’ve done on other clients or stuff you made up.

Option B (Power move): Send a potential client a PDF of designs and mock-ups for their brand / account that they’re free to use, today. Regardless of them hiring you or not.

Seriously. Option A is the norm. Option B is what will set you apart and let a potential client know that you’re serious and capable.

To do this just imagine exactly what you would do for the client in the first few day’s of working on their account. Put it all in a PDF, make a mock-up social media Feed (if you’re a designer you should be able to do this but you can all several different planning apps to do this) with your proposed designs to let them see how it would look and include that too. This will be ten times better than showing them “examples” of your work.

If you’re really worried about them just taking the designs and leaving then you can watermark them, but if they’re a serious company they’ll be really silly to steal your stuff instead of just hiring you and having a constant supply of your work.

Edit: Forgot to add this, a good state of mind to keep while doing this is that no work you do is in vain. If the potential client passes on you, your work has not been a waste. Alter the designs slightly to fit a similar potential client and send it to them. Keep a file of all the proposals you’ve done in categories depending on the type of client. Use and repeat as much as you want and you’ll be developing your skills tremendously in the process as well.