I stumbled upon this article with "best practices" for writing your own LINQ extension methods.
It starts off with some real basic stuff that really applies to any type of extension methods; keep them in a sensible namespace so that users of the extension methods can opt-in rather always get them. I don't think there is always a special need for an extension method namespace which is suggested, but in my experience it is something to strive for.
The rest is good stuff to know. Or rather good stuff to think about because you should already know it. But it is always good with a reminder!