I use Harvest for time tracking and its partner application Forecast for planning work ahead of time. For invoicing I take the Harvest time tracking, export to

I use Harvest for time tracking and its partner application Forecast for planning work ahead of time. For invoicing I take the Harvest time tracking, export to Tiime.fr (my accountant uses it and I get it for free) and generate the invoice there. Harvest + Forecast + Tiime (free) is only about ten bucks a month. As for when and how to invoice, I mostly do retainer work and send invoices at the end of the month. For some hourly work or project work I bill at the end of the project or when I judge the amount of the invoice is worth bothering the accounting team at the client about.