I do scheduling for about 60 people. It is a nightmare some months. My suggestion- set forth a written set of guidelines and do not give in on them.
We post a "wish list' about a month in advance, we allow people to mark off (4) days they absolutely have to have off. The rest is fair game. They schedule what they "wish" their schedule to look like, then we make adjustments as needed.
If someone wants 4 consecutive days off- they must fill out a vacation request that requires approval.
Holidays and weekends are pre-determined and written out for the year. Anyone that wants to switch shifts must fill out a form for that too, that way we have written proof that someone has agreed (both parties sign it).
Hope that helps!!!Good luck.