I was tooling around today in class, tidying up and I found an eraser on the floor. Then I randomly started rolling it between my palms. It turns out to be a great little exercise for the triceps, pectorals, and even latissimus dorsi.
Unlike the isometric version of pressing the hands together, the dynamic rolling action (TM) seems to generate instability and activate secondary muscles in the palms and wrist.
As for the isometric version, it's better to press your palms on the opposing biceps (in the bouncer pose).
Much less effective, unless you are focusing on the wrists (in which case you should oscillate the hands slowly), is just pressing the hands together isometrically. Fitness is a habit. Get it.
The eraser is your friend.