GNU GPL - Copyright helps Copyleft
Make later enforcement easy

Practical rules for proof by reverse engineering
Don't fix typos in error messages and symbol names
Leave obscure error messages like 'Rusty needs more caffeine'
Make binary contain string of copyright message, not only source
Practical rules for potential damages claims
Use revision control system
Document source of each copyrightable contribution
Name+Email address in CVS commit message
Consider something like FSFE FLA (Fiduciary License Agreement)
Make sure that employers are fine with contributions of their employees
If you find out about violation
Don't make it public (has to be new/urgent for injunctive relief)
Contact lawyer immediately to send wanrning notice