Invoice weekly on 30 day payment terms. Offer a bug fix (not new feature) warranty that is conditional upon on time payment of invoices. One week prior to the invoice becoming due give your key contact a gentle reminder that a warranty is provided if the invoice is paid on time. 2 days prior to invoice being due, give them a personal phone call to gently remind them of this again, remind them that it is in their interests to have the warranty.

However, you must negotiate and get signed agreement from the client before you commence work. You can't negotiate terms after you have started work for a client.

Politely and without emotion let the client know that for practical reasons you can't do any work when invoices are outstanding. If such a circumstance actually comes up then politely but firmly remind them that you can't do any more with the invoices outstanding. Don't waver from this.

You should write all this stuff up into your terms and conditions document which the client signs before you do any work for them. Never do any work without a signed agreement. You might want to post to HN asking if any other freelancers are willing to post their terms and conditions documents so you can see what others do. Get a lawyer to check your T&C's.