We did the shirts in Jan. We had done autograph books on a previous trip and wanted something different. I picked up some Hanes tees for under $5. I chose bright yellow for both kids to match- I figured white was boring, pastel not exciting, and dark won't show autographs.
We took along a fat Sharpie and a clipboard to stretch the shirts on for the characters to sign. You must have something hard or they can't sign because the shirt lumps up. I carried it all in a canvas tote on my shoulder, it was very light and everything was easily removed.
Looking back, it was a bit of a pain but well worth it. I wouldn't do it a second time but we had great interaction with the characters, the cms were fabulous, and it is one heck of a keepsake. In fact, the kids refuse to wear them outside the house because something might happen to the shirts. They sleep in them and I've taken lots of photos of them wearing the shirts (both front and back views).