Most people try to put protein in their smoothies, as well as some sort of fibre to fill them up. Good ideas for protein are protein powders, skimmed milk, cottage cheese, raw eggs, soya milk..... actually, you could put any sort of meat in there if you wanted. I know it's a bit 'orrible, but it's not as though you're going to taste it, is it?
Most of us find that a few spoonfuls of psyllium husks, or other kinds of fibre, or anything that swells up inside you and makes you feel full also helps. Porridge oats are a good-un.
There has also been some specualtion that anything which has antioxidant properties will help. Lots of spices do this: nutmag, cinnamon, turmeric, ginger. There are more but you'd have to do a search. These also make the taste crazier, in case you accidentally get a whiff.