Cash flow is mostly about timing. Every day an invoice sits unsent is a day you don't get paid.
Practical tactics: invoice the day work is done, require a deposit on large estimates, accept cards online (faster than checks), automate reminders before and after the due date, and apply late fees after a clear grace period.
Glo Invoice supports all of these out of the box — set them up once and cash flow improves on autopilot.