TRACK
Write the hours down while they happen
An hour you worked and did not record is an hour you will not bill. Memory is generous on the day and stingy at the end of the month: the call that ran long, the evening spent on a draft, the ten-minute favours that add up. Record time as it is worked, against the client it was for, and the invoice is written from a record instead of reconstructed from a calendar.
In Summarly
Start a timer from any page, on your phone too, or log hours against a project for each customer on a week or month calendar. Each project shows what is still unbilled, so nothing you worked goes unnoticed.