To build a very strong freelance portfolio, make sure to showcase your best work. It's very essential to pick your top projects and make them shine by displaying them to your potential customers. It's like you're showing off your best art to your friends.
What do you think?
Leveraging testimonials are great because I've tested it. But I also think you should create a professional website because makes you look serious and organized.
The best way to build a very strong freelance portfolio is by constantly acquiring skills in different fields. As a freelance, there's so much you can learn and offer services on each and every one of them. Pick the one's which you can be a pro in and build it very well.