In addition to placing the mic ABOVE the vocalist's mouth, also try angling it a little, so they're singing across the diaphram a little, not directly into it.
A desser seems like a magic bullet sometimes but a lot of people don't know how it works and therefore how to use it correctly. It's basically a compressor that is acting on an auxilary signal (sidechain). The compressor sees the level on the sidechain go past the threshold, which tells it when to compresses the main signal. So in the dessing function, you want to send your original track through an EQ that EMPHASIZES the "sss" frequencies and use this as your sidechain. The compressor will see a huge level boost in the sidechain during the "ssss" syllables and turn down the volume on your non-EQ'd main track, reducing the volume on the "sss" sylables.
Or fix their dooky mixes.
