I often use the Design By Humans' blank T-Shirt models on my mockups for my entries on Shirt.Woot derbies. Obviously the shirt colors are different on Shirt.Woot then they are on DBH, so I have this file of DBH models, where I have color corrected the shirts to match Shirt.Woot's color swatches. There is nothing technical about how I went about this and it's done just by my own eye, but I have never had an entry rejected using these color-corrected models on my mockups.
One thing that comes in handy when designing shirts for Shirt.Woot or Shirt Fight or any number of the shirt competition sites is having good blank T-shirt templates to display your design on.
A good T-shirt template will give you a really good feel of what your design might actually look like on cotton and seeing it on a shirt helps frame and position a design, and usually gives you ideas on how to improve it or what direction to take it.
My name is Kris Robinson, this is where I blog about art, computers, illustration, design, work, life and other stuff... basically anything that pops into my head.