I use very watered down acrylic paints as a fabric paint rather than fabric dyes and I'm using non-silk fabric so what I'm doing isn't actually Shibori - but that is what has inspired and informed me.
The particular Shibori technique I've been using is called Arashi aka Pole Wrapping. Usually it's done on a diagonal but with this Tshirt, I've done it vertically. I wasn't too careful about making the top of the paint be in an evenly horizontal line which is driving me a bit nuts but I don't notice it much when I wear it so hopefully I will be able to ignore it :)
For those interested in trying this, I didn't even buy any special supplies (eg PVC pipe, sinew or waxed twine).
I covered an old cardboard tube with a plastic bag (so it wouldn't get soggy when it got wet) and used a rubber band to hold each end of the shirt on and used waxed dental floss to wind around. I'm sure twine would be easier on the hands (dental floss is so thin and my fingers were pretty sore afterward) but I like using what I have on hand. When I'm done, I unwrap the floss and wind it onto an old cardboard toilet paper roll to re-use for future Shibori :)
Because the paint was applied by hand to a mostly dry Tshirt, most of the colour stayed on the top exposed layer of the shirt. When using a dye bath or having the shirt very wet first, more of the colour will spread through other layers.
I washed and dried once before wearing. I've had great success using inexpensive acrylic 'craft' paint instead of fabric paint. It does need to be watered down to avoid being stiff or crunchy. I've never used any of the additives you can buy to 'transform' acrylic paint into fabric paint.
This was a plain orange Tshirt and I was hoping for a flame-like effect along the bottom of the shirt:
Check out this great post showing various Shibori techniques: DIY Shibori