I use a pair of small needle nose vice-grips. I clamp the retaining ring with grips, hold the pin against the backing plate and the push in and turn 90*. I have a couple of the tools sold by the tool trucks that I bought when I was still working but a lot of times it was hard to get behind the axle flange with the tool and the grips worked better. The one's I use have an angled nose on them.
