Hi.  My name is Billy Cook.  I'm a singer/songwriter from Dallas, TX. 
The following is a short correspondence I had with Dr. Tim Jameson (an
author of a book for stress injuries due to musical activities).

Begin correspondence:

> 
> writes:
> 
> << I was wondering if you could give me some physiological basis for this
>  problem (like a mini anatomy lesson) and make some suggestions to
>  prevent it.  I suspect that a guitar pedagogue may have comments on this
>  issue on the tip of his tongue; possibly you could give me a referral,
>  also.
>   >>
> 
> Hi Billy,
> Usually, this finger pain is due to irritation of the bone itself at the tip
> of your finger. (Of course without examining you, this is really just a good
> guess.) There's an outer portion of the bone called the perisoteum which is
> very sensitive and has lots of pain nerve fibers.  (That's why it hurts so
> much when you hit your shin on the coffee table.)  When you push down hard on
> the string, you may be irritating the tip of the bone in your finger.  Most
> likely it's not the soft tissues, like the skin itself, or underlying layers
> of tissue.  I'm sure that you've been playing guitar for some time and have a
> pretty good callus formation on your fingertip as a result.  The callus
> formation is your body's protective mechanisms that build up the resistance of
> the skin tissue to the physical stresses of playing guitar.
> 
> So, what to do you ask?  (I knew that was your next question). Well, the sad
> part of this is that the only way the bone will heal is to stop irritating it
> - and you know what that means.  That's right, stop using the fingertip for
> playing temporarily - probably over a few weeks' time.  I suspect that if you
> use other aspects of your ring finger, like the fleshy pad just below the tip,
> there's no pain.  That's because this aspect of the bone is not bruised.  You
> may have to alter your playing a bit to take the pressure off that ring
> finger.
> 
> Let me know how it goes.
> 
> Dr. Tim Jameson

Rest heals, unquestionably.  That's interesting to note that it's bone
perisoteum (containing pain nerve fibers) that's being irritated. 
Playing more pad less tip is good for protection where irritation is
present.  Also, the point that the callous is the body's mechanism for
protection from bone irritation gives a pro-active direction: develop
the callous.

Just in case anyone asks you this question again, I've had great success
in the past with a method for building and maintaining callouses.  Get a
junk guitar (second to your principal instrument) and set it with
strings one or two steps heavier than you would normally use.  Let this
be your warm up guitar.  Then do song prep, improv and performance on
your regular.  Specifically, warm up exercises with descending slurs
(e.g. #5 and #6 on p. 51 of "Pumping Nylon" by Scott Tennant).  (These
exercises are particularly effective when strictly adhering to the
descending slur method detailed on p.14.)

Thanks for a thoughtful answer.