I use Quickbooks Online. I enter my orders as they come in, using the ship/delivery date as the invoice date, then before I start production I run a report of what products are scheduled to ship or be delivered that week. (I now have these reports set to auto-run each week and they are automatically e-mailed to me.) It's also just a great small business accounting tool due to the reports it offers (although I have an accountant friend who hates it!).
In addition, I post each individual order on the wall and arrange them by ship/delivery date, highlighting any important bits (i.e., add extra samples, etc.).
Most of the time this manages to keep us on track.