I have travelled with those couchette sleeping carriages quite often and there is really no art to it. Personally I think it is a very comfortable way of travelling and I rarely have problems sleeping on those, in the contrary I think the slight shaking of the trains makes me fall asleep very fast.
There are maybe a couple of things you might want to consider:
- Security: The problem with night trains is that they stop for a while at random stations in the middle of the night. That is perfect for people getting into the train fast, go into one of the compartments and take the luggage out while people are sleeping.
For this reason you should definitely use both locks (there's usually a lock in the door and then an additional chain lock).
- Short beds. The bed tend to be rather short, which might be an issue if you are tall. In that case I would recommend that you do not take the uppermost berth. There might be less space for your feet due to the curvature of the carriage roof.
- If you are scared of heights, do not take the uppermost berth. That may sound silly, but it is quite high.
- As the train usually stops a couple times during the night at railway stations to be rearranged, and there's bright lights at the railway stations most of the time, I would recommend taking a sleeping mask if light is an issue for you.
But as I mentioned before, there is really not much to it. Enjoy your trip!