I don't understand the first part of your post. Are you planning to launch T-shirts?
Yes, there are tactics to think of ideas. It's not a straightforward way, it's not like "let me come up with ideas". And there's definitely not "aha!" moments. I don't believe in "aha!" moments. You can use design techniques and exercises to come up with ideas, there's no guarantee that you'll have a good result, but you need to keep iterating.