Warehouse employee scheduling

How do you guys schedule your warehouse employees as far as working hours? Up until now we've been contacting them a few days in advance of a day we'd like them to work and they come in and usually work until everything is done.

I'd like to set something up that's a little more professional, but it's difficult since purchase orders come in from suppliers at various (and changing) times and overall volume fluctuates. Even when we think we know exactly when a purchase order is coming in (and we're right), it's impossible to say how many hours we should schedule an employee for in order to receive and ship the purchase order.

How do you guys handle this sort of thing?