Rhubarb on PV days is a good way (or 100g only on PP days)... otherwise, glass of cold water first thing sometimes helps.
What PP/PV rhythm are you doing? He recommends 1/1 these days, contrary to whatever the book says (an amended translation of a French book written a few years ago now...). Also wheatbran can be added. In France, from day 1, we have 1 tablespoon of wheatbran with our oatbran. It gives a lovely nutty flavour to the galette/muffins/bread or whatever you do with your oatbran!
Why not join in our menu thread and we can see what you're eating. I presume you're eating some leafy greenery on PV days (and not just adding, say, a tomato as I sometimes see...)...
But lastly most of us would agree that we just don't "go" as regularly as before, because we're not eating as much fibre...