Great idea! Thank you.
It seems like your request is more about creativity and how to let creativity "do its thing."
Creativity can be a learned skill. I see creativity simply as connecting bits of knowledge that were not connected before or connected in a different way. Right now, create a mental image of you standing in front of two somehow connected objects (choose specific objects.) Now take a step to the left... to the right... up or down... from inside... etc... looking at those two things from a slightly skewed view. How else are they connected? How are they not connected? Is there other objects that can connect them through their dissimilar characteristics? Once you explore that, try it with two objects that you can't immediately think of a connection. When you see an object or hear an idea, try to consider how you can use that object or idea in a different manner than it was intended.
The above sees creativity more as a learned skill, instead of an innate ability, and some ways to further your skill. (Of course, it must be acknowledged that people have different starting aptitudes.) But how you can you maximize and expand your creativity? Well -
this is a highly personalized answer because it will differ from person to person:
1. Know that the majority of your thinking happens subconsciously. Don't hamstring your subconscious by being too busy with things that require intense conscious effort. I write best when a little drunk because I stop trying to force the words out and it tempers the perfectionist in me. No, I haven't been drinking (my employer would have a problem with that!)
2. Set aside time for creative work. I work best in a quite, lone environment. This keeps distractions to a minimum. However, sometimes I will just go sit in a coffee shop with a Moleskine notebook and people watch. I will sit there drinking tea for a couple of hours. Sometimes I write furiously, sometimes I people watch trying to guess what makes them tick, and sometimes I just daydream. I give myself the time to be creative and not force it out. Sometimes the process can be very boring with spikes of rampant activity.
3. Try to keep the same or similar (or minimize how many different sets of) situational environment conditions you experience when trying to be creative. My reason for this is conditional learning. Conditional learning? If you study for a test while listening to Bach, your brain associates that music with the learning and will have a more difficult time remembering it without the music. So find what you feel is the best environmental factors for you to be creative and stick to them. That doesn't mean that you can't change it if you find you are having a hard time being creative in that situation anymore, but only try to switch if you are having trouble. Change is a universal fact of life, but don't try to force it on your life if things are working.
4. Keep learning. Learn all you can about anything that interests you. Or even learn just a little bit about something that interests you when you don't have time to get in-depth knowledge.
Don't skimp on this one. Set aside some time so that you can do this. This is important for creativity. If you, like me, see creativity as connecting objects or ideas in ways that previously were not done - how can you connect objects/ideas if you don't know about them? Your creativity is limited by your knowledge of ideas, skills, and objects.
5. Actively keep a questioning attitude and practice.
Although this doesn't directly answer your question, but hopefully it answers it in a slightly different-than-expected way.
Writing a story about your shirts: How do you do it? 
