"Free" work is literally part of the word freelancer and you should accept that you are going to have to do unpaid work, especially at the start.  The best types of free work to do are the types of work you're really excited about doing over the long term.  Some examples of things you might want to think about doing:

  1. Offering a free high-level analysis for a potential client
  2. Joining meetings with the client to offer perspectives
  3. Writing on your own personal site or blog about topics that excite you or relate to the field you want to do work in
  4. Initial diagnostics like interviews or surveys to help a client assess what is happening in their company

A great twitter thread on this topic:

More thoughts here