The way I do it is go to the USPS site, put in the zip as if I'm looking for a PO in that area... and then it will usually give you the one that it would go to. Then you call the direct # and ask for their postmaster.
The key is doing it soon after it went out though. So you have to keep tabs on your packages. If you wait too long, then they're never going to remember something. One thing I have thought about doing, is getting some sort of sticker, like a big green dot or something. Then you could say... hey, it was a box with a big green dot. That would help get them to remember it. But my boxes are long, so they're pretty easy to remember.