For standard coving, one of these is the simplest tool for cutting 45° internal and external angles:
http://www.thesitebox.com/miscellaneous/wondermitre-diy-miswmdiy.aspx Available in all DIY stores.
If you have patterned/moulded coving, or need to cut different angles, place the coving into the mitre box with the edge of the coving that goes on the ceiling against the base of the mitre box and the wall edge of the coving against the side of it.
It's best to use an old fine toothed handsaw for cutting it, if you have one.