This is probably pointing out the obvious to many of you, but in case you haven't read about it, swapping to smaller plates makes a big difference. Apparently 20-30 years ago we had much smaller dinner plates, and so put less on them. Well, I had to replace our crockery set recently (due to what I believe is called 'wastage' but in our house is more realistically called 'Oh b***er, I dropped another one') and I searched and searched until I found plates that were only 19.5 cm in diameter instead of the now common 25-26 cm. We replaced our entire motley collection, even the bowls, and hey presto, we did indeed find we ate less.
In fact, our food bill has gone down because I am cooking smaller quantities. One son has lost 4kg, I've lost 1kg, and husband is at least maintaining his weight, despite being almost permanently away on business trips.
Success!