Elsewhere, arnie mentions the phrase pig in a poke,, the word "poke" meaning "sack" in this context.

Within my personal experience, this sense of the word poke is never used, and has dropped from the langauge, except in this one expression. My experience may be overstating the case slightly (AHD lists this use of "poke" as valid but "chiefly southern U.S.), but the point remains. Similarly, one never sees the word kith except in one phrase, "kith and kin".

Are there other words or word usages which, otherwise obsolete, survive solely in a familiar phrase that preserves them?

... is another one I've come across. I don't think 'hale' is used on its own though the Oxford Encyclopedic Dictionary defines it as 'strong and healthy'. (It does say esp in the phrase, 'hale and hearty'.)

I think it's interesting to note that these phrases usually alliterate, making them easier to remember. In fact alliteration is very close the the heart of the English language.
According to Chris Baldick in The Concise Oxford Dictionary of Literary Terms, "Now an optional and incidental decorative effect in verse or prose, it was once a required element in the poetry of Germanic languages (including Old English and Old Norse) and in Celtic verse (where alliterated sounds could be placed in positions other than the beginning of a world or syllable)."