To echo the post above, I find it`s helpful to fret the chord or double stop with the absolute minimum pressure required. I`ve tried it out myself to see how I actually do it and find that I increase the pressure as I reach the target fret, squeezing it as I come to a stop - this is what seems to keep the notes ringing out without having to add loads of gain.
Try looking at the target fret just before you
slide too - not your fingers, they are already where they should be - and visualise "braking to a stop" just behind it.
A compressor will help smooth out your playing in terms of volume, but you`ll gain in the long term if you can learn the control to keep your axe singing "clean".
Will