Greasing the bolt is a great idea, I always start with that. And using a good-quality and long-handled Allen wrench!

Making sure that the tip of the bolt isn't perhaps bottoming out in the bottom of the hole can be done by adding a "thin" washer, testing for improvement.

In the days of mostly lightweight 26mm bars, it was not uncommon to see handlebars having been subtly re-shaped by the clamp, such that having slipped once, they would forever slip down to the position defined by the deformation.
It's much less common to come across 25.4mm bars having been deformed by the stem clamp, because the bars tend to be a little thicker.

Removing any lubricant that may have crept into the clamping surfaces might make a difference.