Ask HN: As a freelancer, how do you ensure clients pay?

Reward early payments.

Spending time up front to understand payment terms, calmly, carefully, contractually, means that both sides understand what they are getting into. Nobody would have dared break that word, on both sides. Nobody broke that word. So it sounds like the upfront handshake was either rushed or nonexistent.

Use a contract and set your payment terms clearly; bill at defined regular intervals (with a partial up-front payment).

Make sure there's a late fee, and enforce it! You will almost certainly find clients that don't pay on time, and letting them essentially get away with it will only make it worse.

The one measure I've found that puts honest but lazy clients on the ball, though, is stopping work as soon as payment is overdue and not continuing until they're current (including the late fee).

We use deliverables, milestones, a work breakdown structure (WBS), and statement of work (SOW) contract.

These describe the scope, schedule, cost, payment agreement, refund policy, etc.

http://en.wikipedia.org/wiki/Work_breakdown_structure

http://en.wikipedia.org/wiki/Statement_of_work

When we reach a milestone, we invoice the client for the amount we agreed to in the statement of work.

Fifty percent upfront. Unless it's a smaller project. Then, it's 100% payment upfront. No exceptions.