Use a planner, sticky notes, etc. When I get super busy, I print off a copy of the invoices as they come in, put them on a clipboard together, and write down any info that needs to be remembered.
For rush orders, my white board is my best friend - I write down orders that have to be done asap, in the order I need to do them in.
And a planner works well for keeping the rest of my life organized. If I don't write those things down when I get really busy, I tend to forget.