Well the slave comes through the firewall a little bit and there is a seal there that sometimes goes bad on it.....check for any leaking fluid in the passenger compartment side of the firewall where the clutch pedal is.

Its a first clue the seal may have given out or is beginning to give out and it is fairly easy to replace...couple bolts and a $1.00 part or so.

Also the plunger itself has rubber gaskets around it which may be getting worn.

Slave cylinders really arent that tough to rebuild as long as you have the Haynes manual and the right tools.