A good tuning fork?

[Mike Kurta]

    Lets go one better.  To elaborate on Don's comment, warm your fork to 
body temperature under your armpit for 5 minutes, then using an electronic 
pitch source you trust, adjust your fork by filing at the crotch to lower 
the pitch, or file the tips to raise the pitch until its agrees with your 
reference.  Then warm it the same way each time you use it.  Since body 
temperature stays virtually the same, your fork will be accurate whatever 
the surrounding temp.
    Mike Kurta