You'd normally use the -u flag for this. From tar's man page:

 -u, --update

       only append files newer than copy in archive

so this sequence will do what you need:

# First create the tar file. It has to be UNCOMPRESSED for -u to work

tar -cvf my.tar some-directory/

# ... update some files in some-directory

# ... add files in some-directory

# Now update only the changed and added files

tar -uvf my.tar some-directory/

# Compress if desired

gzip my.tar

For a slightly more detailed look, see here: http://www.gnu.org/software/tar/manual/html_node/how-to-update.html