It's a good point that was brought up. That there is a process, that involves steps. I was speaking with a customer at my shop recently and he remarked to me that some of the younger people in his organization like to just jump straight into the computer. No sketching, no paper. He's old school so and his computer skills are rusty, but he really believes in pencil and paper.
In my opinion, and from my experience, the computer is less of a creative tool, than a simple "tool". Like a hammer. I don't "design" with it, I use it to help me realize designs in digital form. The important thing to me is to get as much of my idea down on paper, sketches, assets, varying possibilities, etc, and wait til I'm really "feeling" with certainty that I can achieve what I have done on paper. Everything prior to the computer serves as a blueprint. After that I pick up my hammer and chisel and set about realizing the design.
Other than that, I try to be very organized. I have a library of extremely organized assets and ideas. I use Adobe Bridge to organize it all. I keep all of my physical sketches and notes organized. Everything is within very short reach. When I find that something is not, I make a special effort to rearrange and get things to be within short reach. The truth is, if you know you need something, but you can't remember where it is, you may skip it.
I keep my desk dead empty. I don't want to "clear it off" when inspiration strikes. I reach for the paper and pencil which are very handy/close by. The less it takes me to "get started" the more likely I will "get started" and also have the best chance of finishing well.
I follow this set of steps for everything I do. Not just t-shirt design, but whatever I set about doing. This process always gives the best chance for looking at every angle of a thing, and ending with the best result.