It's a hard one. Some people, particuarly some age groups and cultures, show their love through food, so if you don't accept said food, well, then you're not appreciating their love.
I'd say perhaps have a tiny amount of each dish she serves and exclaim very appreciatively when you're finished "wow, thankyou, that was such a nice meal, it was so delicious and filling I couldn't possibly eat another thing!" and then just stick with the "Gee, really I'm just too full, but thankyou for offering" thing
35kg lost. (November 2005 - October 2006)
15kg gained again (as at October 2010).
Back to the drawing board - Let's do this thing!
"You can't change the winds, but you can change the sails"
"Reach out and take control of what lands in your lap"