KNOW YOUR TARGET AUDIENCE
If you don't know who your selling to you might be wasting a ton of money printing shirts that will sit collecting dust in your garage. It's important to do your research before you start the printing process. Doing the boring research before can not only save you money, but it might also make you more money in the long run.
(Example: If your target audience is skateboarders ages 10-25 than figure out what their buying patterns are and the types of designs they like.)