Style guides come to us all like death; at the end of a lifespan. This doesn’t necessarily mean that all the work you have done should be forgotten. Smashing Magazine has an article on designing style guides to make our lives easier.
I’d add for the sake of completeness that you might like start this as you progress through your project to save time and energy (and valuable memory cells) at the end. It’ll help you remember what to do if/when the client comes back for more work too. Good times.