After cutting out all of the blocks, I like to iron them (this is a must for the blocks that have had the strips of fabric sewn onto them). Be careful when ironing: some designs (the really plasticy kind) will melt. I prefer to play it safe and iron all of my blocks from the back, and only iron the front side where there is no designs.
While ironing, I sort out my blocks according to size, which helps once it's time to start laying them out.
Laying out blocks for t-shirt quilts is just like laying out blocks for any other quilt. You can visualize in your head how you want it to go, but I highly suggest actually physically laying them out to make sure it actually looks how you imagined (colors go together well, etc).
Many t-shirt quilters just lay out big blocks of tshirts or big blocks of tshirt with borders, but I like the more crazy random look. This is where I the "How to Make a Too Cool T-Shirt Quilt" book I recommended before is helpful, since she gives an explanation of a fast and easy way to cut and lay out her t-shirts to achieve that look. I generally use a modified version of her method.
Next: Piecing together the top