I used to find that stuff like Gatorade worked well for me, although that may have been a placebo effect.
I normally find that so long as I have half-decent pre-workout nutrition, I'll be fine during the workout. If I suspect that I'm going to need something more during the session, I'll bring some powdered milk with me, and if I need it I'll mix it up in the gym and sip on it between sets. Apparently milk + training makes other people vomit, and I find that for a lot of things fitness-related I prove to be the exception to the rule, so that may not be the best idea for you, but as the saying goes, you'll never ever know if you never ever go.
