How to deal with a client that delays the payment indefinitly

[mazoku]

The client is big and well known company. I've finished their requirements. They have tone more that they just asked to be done. I've finished them too. And for a month they've stopped responding or just send me just basic responses.

That is the third job I've done with them and my error was to agree on too much work without intermediate payments, just one big payment at the end. Stupid me to trust them. 

[Theorems]

Threaten legal action and hopefully, the threat will be enough.

[b10b]

Tough break, and there is likely more to the story.  My advice ... Avoid empty threats.  Avoid destructive tactics.  Professionally state the agreement and provide a clear ask that includes deliverables and schedule - reissue or revise as appropriate and make it simple for the other party to resolve.  Beyond that avoid wasting more time and money pursuing dead ends.  "Learning" is the payment for this job, and you can only spend it with yourself.  Invest it in identifying better deals and structuring future agreements in ways that reduce everyone's risk.  Somewhat trite but true nonetheless.  If others are at risk you could consider publishing more facts - but only as a safety message, not as a criticism of other parties.  Integrity and patience,  good luck.

[rgk]

I've been in this situation a lot of times, no one mentioned you can contact a collection agency (you won't get all your money but at least you get some) and small claims court is pretty reasonable (and you don't need a lawyer).

But I agree with @b10b don't threaten, be professional and always have a backup plan. Ask for money up front and set deadlines when you should be payed and make a good contract!