how to cross-build a single application ?

What is the way to properly cross-build a single program
(after having gone throught the 'toolchain' and possibly
even a full 'buildworld') 
from the top-level directory of a FreeBSD source tree ?

right now i do something like

	cd $SOURCE_ROOT
	make MAKEOBJDIRPREFIX=/my_obj_tree TARGET=amd64 buildworld

but i seem to remember that there is a more efficient way
when you want to rebuild only one program or one subtree.
I think i have seen this question being answered from time to time
but i can't find how now.

any hints ?

cheers
luigi