When trying to increase sales of your t-shirt, it's important to know a valuable truth, that people buy into benefits rather than features. They want to know, "What's in it for me?".
As t-shirts are quite simple, they don't have many features, but there's plenty of benefits. You need to think of why people would want each of your designs, what's it going to do for them? What human need will it fulfill?
Take a look at these benefits below:
"Pull the ladies with ease in these tees."
(This benefit presses the 'desire for sex' button.)
"Swan around town like a pimp in this fitted beauty."
(This benefit presses the 'desire for sex, money and power' button.)
"You will make at least 3 friends with this shirt."
(Although it's funny, this benefit presses the 'desire for acceptance' button.)
Now if you don't want go so deep, you can always just have a page on your website or catalog listing the physical product benefits titled, "Why Buy From Us?". The easiest way to find a benefit is to put "which means" right after the feature.
Eg. All Of Our Tees Are:
100% Cotton - which means super comfort and durability.
(This benefit presses the 'pleasure' button.)
Carefully Screen Printed - which means quality reproduction. The design will last the lifetime of the t-shirt with minimal fading!
(This benefit presses the 'save money' button.)
Why not have a look through your own t-shirt designs and find out why people would want to buy them. What are the benefits? How will each t-shirt meet a human desire?