Jane Espenson's blog has a few good tips on writing a spec script:

Your instinct is right, not to use a spec to change the status quo of the show. Don't get the romantically-sparring couple together, for example. You want your episode to look like a typical episode of the show, only better, because you will have more time to work with it than the staff writers do. Think a lot about what the show's actual writers are going to do next.

More advice here.