Some time ago I bought a Raspberry Pi to use as a music player. All music is located on a NAS drive. To auto mount a NAS drive on the Pi when it boots, do the following.
Ensure cifs-utils is installed, it should be by default, otherwise simply write:
sudo apt-get install cifs-utils.
Make a directory to mount to (e.g sudo mkdir /media/NAS).
Edit /etc/fstab (sudo nano /etc/fstab) and add the line that will mount the NAS share folder when booting. In my setup the line looks like this:
//NAS/share /media/NAS cifs username=read,password=read,workgroup=WORKGROUP,auto,user 0 0
The share name, username, password and possibly workgroup will of course need to be adjusted to match the configuration of a given NAS.