Make invoicing automatic. Always.
If you like Zen meditation, manual invoicing is for you. You need to first write an invoice manually. That's by pen, or with Microsoft Word -- whatever. Well, three copies of it: one for the customer, one for yourself (to be stacked in the "unpaid invoices" folder), and one for the archive. Then you need to do your double-entry bookkeeping of this invoice, in the receivable account. Every day, you can go even deeper into your meditation while to go through the stack of unpaid invoices, and call customers to remind them.