We use Visio for this kind of stuff. I don't recall whether you need the "Professional" or "Technical" version these days, but there should be a version available with a bunch of high-level EE symbols. In addition to providing the "templates" for these symbols, Visio is nice for editing block diagrams, which you can then convert to PDF, embed in Word, whatever. If you need something that is not provided, it isn't too hard to create it, starting with an existing template. Probably more expensive than it used to be, but it does produce professional looking results.
-- Chuck Cox SynchroSystems Motorsport Computers Hopped/Up Racing Team