You always have a horizon - on the PFD. Even on a visual approach, you will be glancing at the PFD regularly to check your speed, V/S, etc.
In relatively stable conditions, don't ever hold the side-stick over* Operate the side-stick in a "digital" way by inputting nudges lasting less than a second, and after each nudge let the stick return to centre.
nudge - release
nudge - release
nudge - release
If a single quick nudge is not sufficient, instead of holding the input for longer, input two nudges, nudge - centre - nudge.
So most of the time - in stable conditions - the stick is in the centre and you give it a continuous series of very brief "nudge - release's".
Remember, the FBW is correcting for uspets all the time, so all you need to do is just nudge the datum back to the original attitude if it has been disturbed. When flying manually remember that the FBW is still in - a bit like an autopilot, so you have to work with it not against it.
* In very gusty conditions however, you WILL need to hold the stick over, for example when picking up a wing drop. In this situation, FBW is very disconcerting when you are new to it. If a wing has dropped say 20 degrees, you will need more than a nudge to correct it - FBW will not correct large disturbances. You will need to input and hold full roll command to pick it up. Nothing will seem to happen for maybe two seconds, but then it will respond. As SOON as it STARTS to respond and the wing starts coming up, centre the stick again. Don't hold the correction until you have rolled level because then you will massively roll the other way. As soon as the aircraft STARTS to respond to your input, centre the stick - the FBW will do the rest.
So in summary, to avoid PIO's don't hold the stick away from centre for more than a second at a time. Go nudge/centre/nudge/centre/nudge etc. For very large disturbances you might need to hold a full input for a couple of seconds, but release the stick to centre as soon as the aircraft STARTS to respond.
Nobody ever teaches you this which is scandolous, I eventually taught myself after seeing a video of a Tornado jet. The pilot says "coming left xx degrees" and you can see he just goes nudge nudge on the joystick, each nudge-and-and-back-to-centre lasting maybe a third of a second; the aircraft rolls and holds the new datum - he didn't hold the stick over.
I tried this method on the Airbus FBW and it works like a dream.