I've heard so many different ideas of how other people with narcolepsy take their nightly doses of Xyrem. From mixing it with Gatorade, orange juice, or lemon-lime soda, to simply sucking it up and taking it plain (my least favorite option).
The absolute best thing that I have found to cover the taste, without interfereing with the efectiveness of the dose, is to add a little liquid Crystal Light flavoring. The strawberry lemonade is definitely the best; it really covers up a lot of the bitter Xyrem taste.
It has 0 calories, so it won't interefere with the amount of sleep you get per dose (at least I have never noticed any differences). It just makes it so much easier to take my nightly doses, without having to mentally prepare myself and cross my fingers that I can force it down in one gulp!