Find a tube or dowel exactly (or a hair bigger than) your widest girth, and "long enough" (a few extra inches will do).

Take a 4ft length of TP and wrap it in a spiral around the tube (like an extra half inch showing per turn). Carefully remove it from the tube so it holds its shape, and - This is the reason you need the tube for this - Flip it upside-down (so the outer part of the spiral should be at the base).

Slide it down over yourself, and delicately fold in 2-3 spots over the tip to prevent anything from squirting out the end.

It takes 30 seconds of prep time, but is sooo much more relaxing than scrambling for a wad of TP just as you're about to finish.