For my squadron, you got your name on a plane 1) once you made CAPC/CICO and 2) were high enough on the seniority list. CAG was always under the left cockpit window of 600, DCAG on the right, CO on the left of 601 (even if he was a FO) and XO on the left of 602 (ditto). Most squadrons put the FO names under the CIC windows, but I kinda liked our way, which was NFOs' names behind the cockpit with "Mission Commander" above it.
What constituted 'seniority' got murky once we started getting S-3 transitions who were senior (including the XO), but who weren't yet CAPC/CICO's. Guys who had their letters and been in the squadron for a while were getting bumped by brand-new guys...yet the new guys weren't exactly nuggets, either. So it resulted in some grouchy JOPA meetings for a while.