Ask HN: Consultants and freelancers, how do you handle invoicing?

I'm a relatively new freelancer, and I haven't found a great way invoice clients. My current strategy depends on the client -- some people I email a work summary each week, some I'm in constant contact with while working so I just tell them the number of hours, and some I invoice via a self-hosted Invoice Ninja[0] instance.

Have you found a way to deal with this that you're happy with?