Since I started freelancing back in January I've really trimmed down the number of services I'm using. I guess it felt really productive using so many.
How I work with my clients is always the same; invoice every second Friday and only work on a max of two clients per day (2 half day blocks).
So now I simply create draft invoices and then at the end of every day I add a new line item:
I have an event in my calendar that reminds me every Friday to send any invoices that need sending.