In determining which employees will be laid off, you must avoid making decisions based on factors which appear to violate anti-discrimination laws.
In order to avoid discrimination claims in layoff or recall decisions, base layoff decisions upon objective rather than subjective factors, and keep in mind protected class (race, sex, age, etc.) concerns, assuring that the layoffs do not more significantly affect protected class employees than those employees who are not members of such classes.
You should also consult with legal counsel regarding WARN Act requirements whenever you are anticipating closing or reducing operations at any particular location.