What i do... is take a normal Spray bottle, like you can get at wal mart for like $1.00 and fill it up with the same water you are squirting your truck with, while you are drying it, if certain areas dry before you get to them, use the spray bottle, and wet the area, then use whatever you use to dry it. I use the Aquadry chamois from Wal Mart. Its some kind of super absorbant rubber chamois, and it will not scratch your paint. The reason I use the same water I use out of the hose is because it stands to reason, if the water you use is creating the water spots, it can also be used to remove them. Fight fire with fire, or in this case, water with water.
As for getting water spots off that are already trapped, you might have to do what has already been mentioned. Try using Dawn detergent, which will strip all wax, drying it in sections, or with the spray bottle like I mentioned, and then using a clay bar (work magic) and then finally waxing it. This stuff works. I have a black truck, and am also pretty good at detailing myself.
~CYCLONE_06