A lot of the above advice is very good. About three years ago I converted high quality reel-to-reel recordings to digital format. While we considered these important recordings, they were not family chronicles or anything, just some favorite performances we used to listen to when we were using a tape player instead of a record player. These date from the 60's and 70's and were 7-1/2 inch per second and called 4-Track because the quarter inch tape contains staggered stereo tracks in either direction. Some of the more sophisiticated tape players would sense the end of the tape and reverse direction, so that you wouldn't touch the reel until a complete playback had occurred.
The tapes had been used a lot over an eight year period and then fallen into disuse but safe storage in a reasonably temperature regulated dry shelf space. My father posted them across country but well packaged. I was able to glom onto a reel-to-reel recorder/ player that had been in storage under a radio station that converted totally to digital. I spliced new leaders onto the tapes and listened to them for the first time in a generation. I found the great majority to be in excellent condition, well worth preserving. The output of the tape player was in the correct voltage range to plug directly into the computer's line-in interface. I used Cool Edit software (no longer available) to save the signal into WAVE format. I would check the tracks in order to maximize the signal to noise rate, in other words, I tried to capture the loudest portion of the tracks within the highest dB range indicated by the software. I usually needed to balance the left track with the right track.
The quality of the Decca pre-recorded tapes was generally excellent. That of the London ffrr tapes was less so, which was unfortunate because we had a lot of John Reed era Gilber & Sullivan operettas thereon. I found minor print-through of the music near the tape ends, and frequent tape breakages. These were long recordings with thin and brittle tape. On the worst of these I resorted to 'baking' the tape. There are instructions to be downloaded on the internet, but in general, one sets the oven temperature at a very low temperature, and leaves the tape in, in its plastic reel, for almost a day. Don't let the tape get near any metal, of course, it is to be warmed, not cooked! This seemed to make the tape physically stronger, and I got a good dupe off it with no more breakage.
As far as de-magnetizing of the tapes, the nature of magnetism is such that a magnetized bit of ferrous material does not easily change. The term used is of a 'domain' which requires a certain amount of energy within a certain distance in order to affect the recording. When you are handling a tape cassette, the plastic container not only keeps dirt out, but it keeps anything that has magnetic power at a physical distace from the tape. Reel-to-reel tapes are kept in cardboard boxes which perform a similar function. Remember that magnetic media is wrapped all around itself as you roll up the tape, and that stuff lasts for years. In the case of a digital hard drive which also depends on magnetic media, the hard drive box around the media prevents a lot of proximity of anything harmful, and of course there is a strong magnet right there in that box, driving the pickups. Obviously it is kept at a proper distance from the media so as not to harm it.

Lessons Learned -
1) When properly stored, good quality magnetic media can last for forty years and longer with low signal loss.
2) Even 'high' quality tape recordings may show significan bias changes between stereo tracks and opposite tape sides.
3) Excellent digital transfers can be obtained at home with diligant attention to detail and consistent procedures.
4) Greatest reliability in digital archiving should incorporate multi-media including backed up hard drives. If I had to stake my life on (people of the future) being able to hear a reasonable facsimile of my music, I'd probably put 'em back on analog tape, and put the tape in a dry vault. I think the near future holds great promise of more permanent and reliable digital storage, but CD's and DVD's aren't yet there. The media is bound to change, and, of course, the formatting of digital information has and will continue to branch. Having the stuff on a hard drive maximizes one's ability to re-format it at minimal loss before putting it onto the next type of media. (And of course, there will come a time when the hard drives themselves must be dispensed with as whatever's next comes along).