If anyone has tried putting a Cinelli 1A stem on a set of Cinelli bars, I think they can understand the OP's frustration.

I thin coating of grease doesn't hurt, but I use a wooden wedge angled just for this purpose. You don't need to pry it far to get it over the sleeve. I have found that going in from the roght side on a Cinelli, with the stem rotated back about 90* helps, but that may be just anecdotal to my experiences.

Just last week I faced this with a bike I built for my wife, and had to swap the stem out for a different length after the initial build, so I had to do it twice without scratching the center sleeve.