Contrary to the opinions posted above, digital 8 and miniDV tapes will not last forever. To prolong their useful life span, it is recommended that you fast forward and then rewind them every couple of years (to equalize the tension and redistribute the lubricant on the tape surface).
Check out these web sites:
http://www.nationalarchives.gov.uk/documents/media_care.pdf
http://www.larryjordan.biz/articles/lj_videotape.html
I personally archive all my tapes to DVD. My concern is not as much for the life span of the tape, as it is that one day my camcorder may fail and I will have no way to play them without investing money in a new deck just to play old tapes.
Realizing that DV to DVD will involve some loss of quality, I am willing to live with this.
My workflow for archiving is as follows:
Open iMovie and choose 'make a magic iMovie'. Choose no transitions. Plug in camera and walk away. Come back an hour later and the tape has been downloaded and all the clips are in the time line. I delete the default fade in and fade out at the start and end of the project, then save the project.
Open up iDVD, from the file menu, choose make a magic iDVD from iMovie file (or something like that). It burns the movie to a simple DVD (no menus etc).
Although this takes a couple of hours of computer time, it only takes a few minutes of my time. You can process a tape when you are not doing other things with the computer (as you go to bed each night for example).
I have tried to make magic iDVDs directly from the camcorder, but have had a few crashes and problems with this, so I like to get all the footage in an iMovie project so I can check it quickly before going to burn the DVD.
Also, keep in mind that if properly stored, high quality DVD disks should last a long time. However, if you buy the cheap disks or if you do not store them well, DVD disks will also become unusable after a time.