Can't do it, huh? Now ask a News Director to name all the people who sent tapes last month.
If you've sent a tape more than two months ago and haven't heard anything, send another. Situations change, NDs can't remember every tape they've seen, and you probably have a better tape anyway. So stick another tape in the mail. Your cover letter doesn't even need to mention that you've already sent one.
Now, if you've gotten favorable feedback from a ND, you should be sending a tape every two months anyway, even if you never hear anything. When the same names keep crossing a ND's desk, then he might start to remember you.