Is there a particular song that you would use instead of tapes, Linda?
e.g. When I was warming up yesterday, I sang "Dirty Old Town" in every key from Bb up to Bb and that was what got me thinking. Say a nicer piece that would lend itself well to the exercise and gently increase the range over time.
The tapes are a great idea but I know me and I wouldn't follow through with them as I should.