I'm currently using empty gatorade bottles (about 1 liter each). I put in 2 packs of crystal light, some BCAA, glutamine. I usually drink one during cardio and then carry 2 to work. Then mix one up later that night to have next to bed. That way i know i'm getting at least 4 liters a day.
It really helps to have a container of fluid around at all times.