From the ALSA wiki
Hints for Using LADSPA Compression
Start with the compression ratio around 4-5 (that's plenty - it may be more than you need but that helps you dial in the other controls). You won't actually hear any compression until you turn down the threshold level, so do that now. Gradually turn it it down until you start hearing (and seeing on the meter) some gain reduction whenever the voice is actually singing. When you like the sound, add some makeup gain until the overall level is good.
The default attack/release settings are maybe slow for vocals, so I turn them down a bit - attack to about 20, release to about 150.
So far I have not needed to tweak the "knee" parameter.
For my own stuff I don't tend to like really obvious compression except as an occasional deliberate effect. So once I've got a sound I like, I then tend to reduce the compression ratio gradually until I think it could maybe use a little more, and at that point I leave it alone :-):-) Maybe come back to it later and see if I still like it.
But if I'm trying to sound like pop radio, I might go the other way and set the compression ratio to something severe like 10:1. There's some good guidelines to be found all over the net here and here.