I used to think that the XYZ axes in virtual furniture were relative to the items themselves. But a simple test showed me otherwise. Try it! Change the rotation of your furniture and you will see that Z is always up/down, no matter whether you tilt the furniture to its side, and X and Y always follow the E/W and N/S orientations. So let’s see how we can know ahead of time how to adjust our position without fumbling around awkwardly during a scene.
The first thing to memorize, of course, is that the Z axis is always up and down. You can remember this because it is so basic that the Z is at the end of the XYZ list. That may sound dumb, but it works for me. :D
For the other two axes, I recommend you sit on the furniture and open your minimap (Ctrl-Shift-M). Now move your camera until it is facing due north in your minimap while keeping the furniture front and center in your screen.
The second thing to know is that the Y axis is always north-south. Plus (Y+) is north and minus (Y-) is south. This makes sense, because when your camera is facing north, plus (Y+) is towards the top of the screen, and minus (Y-) is towards the bottom. I remember that the Y axis is N/S because it’s the second most basic one, and therefore the second from the end of the list. ;)
Third and last, the X axis is East-West. Plus (X+) is east and minus (X-) is west. This makes sense because, if your camera is facing north and you read it from left to right, X+ is forwards and X- is backward. I am reminded that the X axis is E/W because it’s the hardest for me to remember, so it’s the first on the XYZ list.
This is easiest if your furniture is oriented due N, S, E, or W. But what if is facing NE, NW, SW, or SE? Now you will have to adjust both the X and Y axes to get the same result. With a little practice, you should get the hang of it quickly, though. And I suggest you DO practice before your roleplay session so that you will have one less thing to worry about and detract from your total enjoyment of the scene! :)
Finally, if your memory is as bad as mine, fear not! if you are using my handy-dandy adult furniture guidebooks (https://marketplace.secondlife.com/stores/217863) you will find axis guides on almost every other page to remind you in case you forget.