If you don't want to loose ceiling height, you will need to remove the celing covering to gain access to the floor joist cavities.
A product we've had great sucess with is Roxul (http://www.roxul.com ... read more
John,
Are you referring to insulating the ceiling of the garage (i.e. cold floors above)? Will you be dropping the ceiling or are you looking for options with minimal disturbance?