How are you storing your soap? I have 3 or 4 bars going in the shower right now, none of them are remotely squishy or messy.
+1
I couldn't possibly see how a bar of soap gets squishy in the shower unless you leave in in some sort of un-drained depression or directly in the spray of the water from several other people taking showers.
If you have a quality bar of soap, take it out, place it in a dry place, don't leave it soaking in the shower.