Once a year we plot out "tracks", senior to newest person. Tracks stay in place for an entire year. We work every other weekend and during two weeks, there are 4 monday's and 4 friday's, we must sign up for 2 of them.
Seniority is the key for us, when choosing between 2 people, but there are two gate keepers who work on schedule changes to ensure everyone gets days off, and finangles switches, one is new, one senior.
We NEVER have a problem with the new person being crapped on, we just don't treat our staff that way, seniority takes places for the yearly track schedules, vacations are first come, but we ALL try to switch schedules so no one is told no.
Also, no ones schedule is just "switched" with out their consent. If there is a problem with one person not doing their part, the manager gets involved and negotiates.