A few notes:
1) I used 330 Ohms and 4.7k Ohms on the preregulator for a couple of reasons:
A) Less current flow in an area where good voltage regulation is not a requirement.
B) That ratio of values will result in the output of the preregulator to be about 17.8v above the regulated output. 50v - 17.8v ~=32.2v; your regulated output target is 15v, so you wind up with nearly equal voltage drop across the two regulators, and practically the same power dissipation. This makes managing the power dissipation requirements less difficult.

You did not mention what your current requirements are; I merely assumed 20mA. Even with that light load, and 10.4mA through the output regulator, and about 3mA current through the preregulator resistors, you wind up with ~500mW power dissipation in each regulator.

Note also that my R3 selection is textbook; since Vref (voltage from OUT to ADJ) may be anywhere from 1.2v to 1.3v and still be within specification, and you need to have a minimum of 10mA current flow from the output to ensure regulation, 120 Ohms guarantees a 10mA current flow.

 

Right, thanks for the advice Sgt. I will swap out the resistors I chose for your recommended ones as soon as I get the time =]

 

Pre regulator: Bear with me, I don't know how to post a scheamatic. Place an N- channel fet in series with the input of the 317. Run a resistor from the 50 volt source to the fet gate. Place a zener from the gate to the regulated output of the 317. The voltage rating for the zener must be a little greater that the dropout voltage for the 317 plus the turn on voltage for the fet. The resistor can be a fairly high value ~ 100k. put a .1 uf cap or larger at the input to the fet to ground. The fet will preregulate the input of the 317 to the point at which the zener conducts. Reverse for neg supply