It depends on the dye of the discs you're burning. When buying blank media, make sure you know the Media ID which identifies who manufactured the discs. Taido Yuden is considered to be the best quality media available, followed by Mitsubishi Chemicals and TDK. DVD+R (plus) media is superior to DVD-R (minus) media, and for archival purposes you should burn at a slow speed and enable data verification. (Or check the files' CRC32 against the official CRC32; if it matches then create SFV files.)
Here is a list of good quality media, their MediaIDs, and where to buy them.
Look after the discs by storing them away from heat, humidity and sunlight. I don't tend to use archived discs, but I'm not sure whether that affects the lifespan of a disc; to be on the safe side, I think it's best not to use them. (They are, after all, archived.)
Archiving on hard disks are expensive; you get a much better £/$ per GB buying blank media (even the expensive Taido Yuden discs) than buying HDDs. If you were to archive on hard disks, you should transfer the data on and disconnect it from the PC and storing it safely. (You might as well as buy blank media then!) Hard drives will die faster than blank media if it is constantly being used/spinning in an environment that isn't particularly clean.
Jam it back in, in the dark.
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