There are two high cost inputs to making shirts: materials and labor. Therefore the opportunities for reducing the cost of goods sold are going to come from reducing the cost of materials or reducing the cost of labor.
To reduce the cost of materials, you can search for bargains on fabric, switch to cheaper fabric, negotiate harder to reduce the cost of your inputs, or try to buy your materials in bulk to reduce the unit cost.
To reduce the cost of labor, you must either find cheaper labor or find more efficient ways of using your labor. If you have sufficient scale, you can outsource the manufacturing of your shirts to low-cost-countries (as many of my entrepreneurial friends often do on alibaba.com). You can also try to find innovative techniques for reducing the amount of time it takes to create a shirt (but be careful not to let the quality of your product fall!).
Lastly, I want to remind you that reducing cost is just one way to increase your profit. If you are using fancy materials, you might also want to try a price increase and see how that affects your demand.</body></html>