I use a modified 3.5 method. *EVERYONE* in the party, PCs, NPCs, cohorts and followers, special mounts, special companions and special improved familiars *all* count towards the number to award xp to.
So - instead of having that party of four that has 4 cohorts, 4 animal companions, mounts or familiars being considered a party of four and the same as a party of four with no hangers-on, they are considered a party of 8+.
There is *ZERO* reason why cohorts should not be counted in the party total when determining xp. I don't use the "cohorts get 2/3 xp of their leader" rule. Why should a cohort get 2/3 xp while a PC who is two levels lower than another PC get full xp?
Basically - a bare party of four has a harder time than a party of four with additional muscle in the form of cohorts'n'followers and/or special mounts and companions or improved familiars, so they should get more xp for the same encounter.