2. Put the front half of the list into one half of the trial, and the back half of the list into the other half.
Generalizing this to more than two groups is straightforward. This algorithm is mentioned sidethread, by sterlind, with the (meaningless) modification of splitting the list even-and-odd instead of front-and-back. As I mentioned there, you can only do this if the list of participants is fixed before the beginning of the study, which is not in general the case.
