Water

Arguably the most important element (save oxygen) in the life and well being of all living animals. It cannot be stressed strongly enough that extremely clean water be provided in a proper amount and manner to your veiled chameleon.
These animals drink a lot of water, and they won't do it out of a dish or bowl. Water must be provided via a misting system or drip system...preferrably both.
Being arboreal animals, old world chameleons drink the water that comes to them in the trees. It is assumed that rain and morning dew are the main sources. Veiled chameleons also eat plant matter, which provides some moisture as well.
All of these sources are extremely clean with no fecal matter or decaying plant matter like you might find in a pond or stream.
Therefore, we must guess that they need to be provided with exceptionally clean water. It is for this reason that waterfalls, fountains and water bowls are unacceptable in a chameleon enclosure. Only fresh, clean water from a drip system, or misted onto the leaves of the plants in the enclosure will suffice.
I personally use a dripper and hand pump mister. The dripper sits on top of the enclosure with the small hose coming down onto a vine in one corner of the habitat. This "drinking station" never changes location and the chameleon knows where to go for water (usually first thing in the morning after his initial misting).
The mister I use is one that you find at places like Home Depot. These are often used for pesticides. You pump air pressure into it by hand and it releases a fine mist when the trigger on the "gun" part of the hose is squeezed.
Misting should be done for 30 seconds to a couple of minutes two or three times a day. It is important during these sessions to get the plants and branches quite wet, I can also get the cham pretty wet. Be careful to not soak babies as their tiny nostrils can get clogged and they can aspirate. The water should be at least room temperature....I find I get a much better reaction if the water in the mister is hot (it will cool a bit as it sprays out). It is also important to let the entire enclosure dry out between mistings. You do not want an environment that is constantly wet. Again, we are trying to simulate an arboreal habitat where there is constant circulation of air, and rain and dew evaporate quickly.
The dripper drips at a rate of about one drop per second and lasts most of the day. I have a Rubbermaid collection container in the bottom of the enclosure to catch the water. It is a good idea to rubberband some netting over the top of this container to avoid drowned feeders and keep your cham safe. It is also a good idea to clean this container, the mister and the dripper fairly often with a 10% bleach /water solution and rinse very well.
Drainage systems have been devised by many keepers and are a good idea...particularly if you decide to employ an automated misting system.