Give your taps a service, check all seals, replace if neccessary with a tap kit (but if if you look after them they should last for years...) and lube up all the moving parts.
Fill a keg with a PBW or sodium percarb and fill the lines and let it sit overnight.
I couldnt believe the crap that came out when I did this last week, and they honestly looked clean before I did it.
Well worth doing, especially if you havent done it for a while.