I get them back on using tweezers. First I clamp the case so that I've got both hands free (if you haven't got a clamp then a big blob of blu-tac on a table will hold it well enough to get them on).
There's a tiny groove on the pusher shaft and if you use your tweezers to sit the ring in it it should stay in place (without trying to push it on). Use your free hand to guide it. Once it's sitting there it's just a matter of applying pressure in the direction of the gap in the ring to pop it back on. You can use the flat end of your tweezers. The main thing is to think of it as a two stage operation - positioning and pushing.
It's really important that you do this on a big table with no clutter on it as if they end up on a rug/carpet then they're a real pain to find.