Keeping it in liquid form seems to be a bad idea. Check out alt.drugs.ghb on Usenet. The GHB solution will go bad, or more accurately turn into a gel. Store GHB in powder form with a drying agent.
GHB solutions can grow moldy if the concentration is low (1 gram/5 mL).
Try making a 2 gram/5 mL solution and keeping it refigerated; it should be fine for a long time.