Google has made their Common Lisp Style Guide—presumably from ITA—available to the public. As with any style guide, almost everyone will find something to disagree with but by and large it appears to be a very reasonable compendium of best practices. One of the things I like about the guide is that it gives a rationale for each of the rules. That makes the guide feel more like “best practices” than a diktat from the suits.
ITA, of course, is the canonical example of a commercially successful company that used Lisp to solve a large, complicated problem so their take on best practices deserves the respectful attention of all Lispers. If you write in CL, you should take a look at this guide; it’s got lots a useful ideas.