I am working on a t-shirt quilt now and not using a pattern. Many of the ideas above are really good advice. I will tell you what I've learned to do and not to do.
To do: Decide what size you want to make and what size your machine will accommodate when it comes to quilting. I told my friend to provide 20 t-shirts. I ended up with a 25 - 12.5" squares. Some were squares made up by putting four logos from the front of four shirts together into a 12.5" square. I also made a 2.5" sashing and 2.5" border. This quilt is now almost the size of a king size quilt! Consider making a smaller quilt...say 10 - 15 t-shirts.
To Do: Use the correct stabilizer. I actually purchased stabilizer that was too stiff and used it for about 1/2 of the squares. The other half of the squares I used a lighter weight stabilizer. Now the quilt is stiiffer in some areas and just right in others, making it hard to quilt.
To Do: Use low loft batting - T-shirt quilts are heavy to begin with...Low loft will make it easier to work with in your machine.
To Do: I am seriously considering tying this quilt rather than quilting it, since it is so big and I am working on a manual Singer 301A. I don't want to mess up the backing and batting and make it pucker because I can't keep my quilt laying flat in my small machine when quilting it. LA is not an option because the lady who wants it doesn't want to pay that much for it.
Just my 2 cents worth.