I promised more information on joining cotton in this post on making strong joins ... and Jen was kind enough to say she was interested! So here goes.
When I join cotton, I spin the yarn down a little finer (if I can -- or if it broke, it is usually finer than the other spinning ;-)). Then, I lay the end of the yarn over the cotton sliver or puni, usually at right angles to the sliver/puni, and start adding twist while pinching the yarn end and sliver/puni together. Cotton is so fine, the yarn grabs at the unspun cotton as twist enters the join area, and a good join is soon formed.
That said, you can join cotton in the traditional way, laying two unspun ends over each other and drafting together. It's just that with my cotton spinning, joins usually happen when my long draw draws things out too fine and the cotton snaps. And there is just so much twist in my cotton singles that finding a thicker spot to unspin usually isn't feasible.