here's how it worked:
i didn't want to buy anything new to put over the fireplace because we have a LOT of pictures, and most of them are in frames i already like and want to use. did a little bit of browsing for ideas, and this is what i decided to try:
pick your favorite thickness of painter's tape, and put it in a straight line. i eyeballed this, but if you wanted to be awesome you could measure it. problem is, houses never seem to be straight. you can place the tape exactly the same distance from the ceiling, the mantle, the floor, or whatever you choose, but chances are it won't look straight when you're finished. you could always use a level, too - but, i went with good ol' eyeballs. i did measure when i got it where i liked it to make sure it wasn't a crazy ways off from the ceiling trim (just in case the husband was looking).
pick a spot off center and start hanging pictures that touch the tape. leave the same amount of space between them.
hang a few, pick out a few more frames from the bin, and hang a few more.
and, this pic is just to keep it real. the pile on the right is a bunch of unpacked-but-nowhere-to-live stuff. at least it's smaller than the other 3 piles we've had around, and now there's only one pile.