It helps a lot to do everything s l o w l y and to try to only change one thing at a time. What really matters is tracking the localiser and glideslope; speed and trim are also important but secondary to this.
Monitor trends. If you need to make an adjustment to pitch/power, coordinate it in yaw manually, keep the wings where you want them with aileron, let everything stabilise, *then* trim (if you feel the need). Trying to be in perfect trim absolutely all the time is a) unnecessary and b) impossible. Again, it’s the path through the sky that is important, not that you’re a little bit sideways or holding some rudder or aileron input temporarily. Putting most of your concentration into one aspect (trim) will generally lead to lower performance in other areas (approach tracking, speed control). It’s OK to be out of trim for a bit!
Once established on the approach, unless it is really turbulent and/or windy, it shouldn’t take much in the way of adjustments to carry on straight down the ILS. Small corrections, made in a reasonable timeframe. If you are using a FD, nail it.