The file /etc/fstab defines the various mount points and their parameters (see the Configuration files and directories page). One key attribute is the umask. This is a set of octal values that detemine the permissions of a mount point. It maps to the permissions as follows...
rwx rw- r-x permissions 111 110 101 binary 421 420 401 7 6 5 octal 000 001 010 umask (binary inverse) 000 001 020 0 1 2 octal umask
So to define a mount point with those permissions, the umask in fstab must be 0012 (in most cases you can ignore the leading 0). For a full description, see this Linux FAQ.