Freelance programming and copyrights transfer agreement

nacho66

Hi guys

I've just started doing web development contract work, mostly for web dev agencies (not directly for end-clients, who in the end pay for the website). I wonder about what are the best practices, contract-wise, to secure myself from not getting paid for my work. I did contract work once, 2 years ago, and I never received part of the payment. I've read about copyrights transfer, which would allow me to transfer copyrights to the web agency after all payments are sorted. But not sure if that's THE thing to do and if there's more into it. Can you advice?