Hola Booklady, jer here.
Judging by your posts of late I can see that you are preparing to tear this country up from top to bottom

, yeeehaaaaa!
Might want to see the following thread on Santiago de Compostela, full of ideas and cool stuff:
http://www.multimadrid.com/cgi-bin/jersbbucgi/ultimatebb.cgi?ubb=get_topic&f=16&t=0000 06 As for the best way to get there, I think the train would be a wise decision since it is a longish trip (7-8 hours) and you will want to enjoy y the scenery.
CLICK HERE for the RENFE time schedule and fares from Madrid to Santiago.
If you prefer the bus (I often do but for shorter journeys), the ALSA bus company makes the trip from Madrid (and other cities). The trip will take between 8-10 hours according to them, depending on how direct the bus goes. Check out the timetable at
www.alsa.es/44/horarios/hor_nac.html (

warning: it may take longer for the ALSA webpage to open than the actual trip to Santiago itself

).
Flying seems a bit excessive and you will miss the scenery. Flights are not very expensive however, Iberia has some good Autumn deals good through mid December at
www.iberia.com/iberia_en/home.jsp (you must book online for the specials). I did a search for a ticket Madrid-Santiago de Compostela-Madrid leaving on Dec. 14th and returning on Dec. 17th and got a great price of 15.566 pesetas (about $80 USD).
Other carriers to try for Madrid-Santiago flights would be
www.spanair.com and
www.aireuropa.com Saludos & happy trails
jer...