I have started to throw 1/2 Tablespoon of psyllium into my smoothies in the morning. (The plain ordinary kind.) I also throw in chia and flax seeds. Those help bulk things up.
I also found that pumpkin is a thickener for me. It works for me in pumpkin bread and muffins. The problem for me is that both use sugar, so I have to watch that I don't eat too many for that reason too.