Rate them.
I'm a ratings nazi!
I rate everything, even resembling a mere hint of worthiness for the purpose of posterity.
Use whatever rating system you prefer, but make sure you have a well defined system.
I use the number system available in my software(1-9, with 0 a non rating .. ie. zeroed out).
The traditional method is to use star ratings .. but each to their own I guess.
I rate images down to a level 5, of which will be pretty crappy in quality ,.. which could mean out of focus, motion blur, over/under exposed and all that .. but the rating will be allocated purely for one simple reason.
Personal value.
I then review all images after the year is up(when I can remember) .. and go back through them to delete whatever I can.
the rating system makes it all easy, in that if I haven't already done so, I use the rating view/filtering tool to locate any non rated images.
if the image is not rated, it's immediately deleted.
Then comes the technically hard part.
deleting the rated images.
Like I said, even tho the image is technically crappy, and rated a 5, I might still keep it because of some intrinsic value(eg. kids first step .. crappy image of a UFO .. etc
)
But I may also delete technically good images.
Say I shot a series of images(bracketed or something) .. and they all come out sharp and well exposed, and all that.
I don't need 10 of them, even tho i might rate them 9 or something like that.
So I may delete some of them .. maybe half or so. There is no set formula. The individual image is the defining reason for keeping it or not. Is it different enough .. etc.
I'm currently luckier than you Mike in that my drive is only 1.36Tb full .. which is exactly half the usable 2.72Tb that is supposed to be a 3Tb drive.
But if I didn't delete ruthlessly, I'd probably need at least 3 more drives by now .. one more to keep the other 1.5Tb's of images that wouldn't have been deleted, and another two to keep a second copy of each image as well!
good luck with it all .. and above all be ruthless with them!