Matt, I use Enco surface plate cleaner. Its cheap, and a bottle will last a life time.
I keep my big one covered with a soft clean cloth and out of harms way. The smaller lower grade check stand I usually cover with a piece of paper towel to keep dust, grinding grit, and paint over spray off.
Soft metals like aluminum will transfer onto the surface of a granite plate or check stand after its used a bit. I clean this transfer off with a non abrasive pink eraser.
I've heard that a light coat of thin non-detergent oil helps to protect the surface.
-MB