I think ads are the best way unless you have a large social media following. If you have alot of followers, then you can promote your shirts to them. If you have a website, you can promote it their.
Promoting Shirts can be a pretty hard thing to do for free.
I remember there was one instance where someone created a shirt on TeeSpring about gym teachers and ran an ad campaign for only a few days and he made TONS of sales! So running ads can bring you a good ROI if you do it correctly.
You can run ads on Facebook:
Facebook is great platform to run ads to, because there are a billions of people using facebook daily and if you target the right audience On Facebook for specific niches you could be making a lot of sales. The great thing about Facebook it allows you to search for a specific people with specific interests and habits which makes it easier to find your costumer by looking at what the things that connects them to your brand or idea. You
Can run Ads On Amazon AMS:
AMS Ads is one of the best ways and strategies to promote your shirts or even your brand, because it gives you the opportunity to show up your brand or shirt based on the search terms that costumer uses to look for products. AMS allows you to put your shirt on front of thr costumer that is looking to buy a product, because most of the costumers that searching on amazon are looking to buy.
Rund Ads on Instagram:
Instagram is also a huge platform that has a millions of users all over the world and it is also owned by facebook. Put in mind that instagram is mobile friendly so put your ads and contents based on that to show up the best results to the costumer. Also, instagram ads are very simple to run.