I know which PO delivers my mail, in this case I would call and speak to a superviser or the postmaster.  Occassional delivery errors are to be expected, but your situation appears to be more than I would consider acceptable (once or twice a month, not several a week).

Online might be a possibility, last time I was having regular problems I was not online regularly.

Sounds like someone really needs to look into what that mail carrier is doing... I wonder how much of your mail is ending up with other people as well!  I'd definitely go in to your local post office and talk to the supervisor, and I'd do it right away!  You can probably find out which P.O. delivers to you online, or by calling the 1-800 number listed on the website. 

I know that I can always tell when my regular carrier is not the one delivering the mail- things get crammed into the box or the box is left hanging open, packages are tossed onto the porch or notices left instead of someone knocking, the mail comes very late, etc.  She always tells me to let the P.O. know, because sometimes they have substitute workers that aren't up to par, and the supervisors need to about hear it from the customers.

One thing you can do is put a sticker on the inside of your mailbox where the carrier can see it. Put your address on it and list what names get mail at that address. It will give the carrier one last way of double checking the mail and delivering it right.

1-800-ASK-USPS or usps.com