There's a window after every purchase where a client is genuinely happy and open. It's short - next day, maybe a few days out, gone within a month.
If you only ask when work gets thin, you're always going to miss that window. The desperation isn't in the words. It's in the gap before them.
Every 45 days - not because it's magic, because it means you're never more than six weeks away from somebody's window. Automated, consistent, whether you're slammed or staring at an empty week.