Just to add to what others have said:
You may consider keeping genetic artifacts. If you have a letter in an envelope, don't just keep the letter and toss the envelope! ToTheLetterDNA
has been around for a year or two. They will try to extract DNA from an envelope flap, stamp, or other artifact, sequence it, and provide it in a file that is useful for genetic genealogy. Even if you aren't into genetic genealogy yet, you or a relative may be someday. (But it's a ton of fun -- I highly recommend it!)
ToTheLetterDNA is too pricy for me right now, and I don't know of other players in this niche area, but probably their price will come down and other players will come in. When that happens, I'll be happy to have kept the stuff.
If you have a lock of hair, it has been said many places that only mitochondrial DNA can be extracted unless you have the roots attached to the hair. Mitchondrial DNA can be useful regardless, but anyway this seems to be a myth -- I guess it's hard to extract nuclear DNA from hair, but possible:
And maybe 20 years from now, it will be easy. So hang on to these artifacts!