Shamrock Airport Express: (970) 482-0505 
Bus service is available between DIA and Hilton Fort Collins.
Reservations are generally not necessary, but are recommended by 7pm the night before to be guaranteed a seat.  At DIA go to the ticket booth on level 5 in the main terminal.  http://www.rideshamrock.com/rs_ax.php

Rental Car: Make reservations directly with rental company for pickup/return at Denver International Airport.  There is no fee for parking at the Hilton Fort Collins.