I don't know the "best" way, but:

1) see if someone is already doing it by issuing a GOOGLE search for your list's messages (unauthorized archive?)

2) subscribe an additional userid in MIME DIGEST mode

3) do monthly 

GET listname LOGyymm
OR
GET listname LOGyymmA
GET listname LOGyymmB
GET listname LOGyymmC
GET listname LOGyymmD
GET listname LOGyymmE

/Pete