Focus on the composition, don’t make things as big as you can just because you can, keep your design limited to a central area and rather have it packed instead of spreading it around. Rather have high contrast so you can easily see and recognize it even from far away blab la bla simple stuff . Always keep basic composition in mind so like put the thing people are suppose to focus on on the top / in the horizontal center and let it kinda flow so the viewers eyes are guided throughout the design rather than having to jump around all the time. Think about color as well, the color of a shirt might look good with the design, doesn’t help tho if it’s a weirdass color nobodys gonna wear. Colors should be simple and, most importantly, should harmonize. Sometimes a single color is enough to make a great design, if youre using different values n stuff. Also material and Print type matters a lot. Just look at how the prints look on the type of shirt youre using so you can use it in your advantage instead of having it look shit in the end. Also keep in mind how people are gonna look like wearing it because a flat laying shirt doesn’t look the same as the same shirt when someone is wearing it, also the fit varies a lot and affects design a fair bit as well. and like rather put stuff on the breast instead of the stomach. Don’t make it too complicated. Simplicity is key.Use different fonts and sizes. Pretty self-explanatory, just make it look less boring. Use vector graphics.