A concept rather than an occupation, i.e. following more than one revenue earning pursuit rather than a single lucrative job. It has always been around, but portfolio careering has seen a swelling of the ranks since the credit crunch and resulting redundancies. For a lawyer, one would have thought, a string to the bow could be some form of legal services, and it's not a bad time. Under client pressure to increase efficiency and cut costs, more of the large firms are looking for alternative ways to deliver some routine legal tasks, and talking of outsourcing to contract lawyers. The concept of contract lawyers and 'virtual' law firms is well-established in the USA, and is most definately a growing feature on a changing landscape here. Then there are the more traditional part-time incarnations of locum and consultancy work.
A portfolio career is not going to work for everyone, and there are various considerations in asking yourself if you are suited to it. What you are good at and what you enjoy are going to be key to branching out. Try listing your achievements as a start. Are you a self-motivator - not all of us can provide our own incentives. This leads into adaptability, willingness to learn new skills, disciplining yourself to work in segments, to learn technology, to think like a freelancer, to have the energy and confidence to market yourself, to rely on strangers within a network rather than the institutional support systems of a law firm. What is important to you, e.g. money, creativity etc, and can you afford it, to trade the lucrative salary, at least for a transitional period.
Redundancy can be the key in many ways, not only the enforced push into the unknown, but getting employers to pay for courses and counselling to find out where your talents lie.
Also, see the book 'And What Do You Do? 10 Steps to Creating a Portfolio Career' by Barrie Hopson and Katie Ledger.