Redesign the connector
The obvious, answer if you had the option would be to design the plug so that either:
- the orientation doesn't matter
- it's clear which orientation must be used
Modify the housing
If this isn't an option, the next best thing would be to make a modified cover which forces a given alignment. This is could be done by selecting an asymmetrical shape that only has a single 'correct' orientation, or by including some aspect of a shape that stands out and clearly has to fit in a given position for the connector to be inserted. You can see what I mean by looking at the raised top section of the trailer plug and socket below:
Visually mark the orientation
This is typically done with either a red dot or line on the male and female connectors. I can't find an example now, but I've also seen it done with a line on each side with one in blue and one in red - so that a marker is always visible. Ideally I would make the red one a double line and the blue one a single line, and make each tactile by either raising it above or lowering it below the surface.