Screenshots of some of your best posts is a good start. I’d also recommend adding some analytical data. It’s not enough to just show that you can make a good post, but what IMPACT did your posts make? Do you have data that shows an increase in traffic/conversions based on the work you put into managing a social media account? Do you have previous employers or clients you could snag a testimonial from? You could also, with permission of course, create a case study on a client or business you worked with demonstrating how your social media strategy benefitted them.