I was the executive director at a small non-profit. Our Policies and Procedures required ANY employee who worked over time had to have a signed slip from their supervisor stating why the OT was needed and how many hours it involved Before working the OT.
We required in our P &P's, that if employees wanted to switch shifts or days, they could not do it if it ran someone into OT. Switching required prior written approval as well. On holidays, staff who worked received holiday pay, (time and one half) plus their regular pay for the day. If you were scheduled off on the holiday you were paid time and half for your holiday pay. Sick leave accrual was capped off (how much you could carry over) and you lost anything not used after two years. Annual leave had to be taken each year or permission to not use it given in writing. Any time not used could accrue up to three months and be cashed out at when the employee left.
Employees could donate sick and annual leave to other employees.