There are several existing threads that address this question. Go back and look through them in the food forum.
I personally do herptivite twice a month and repcal w/D3 once a week. I also supplement with collard greens for veiled and a very nutricious gutload for the feeder insects.
There are about as many preferred dusting schedules as there are species of chameleons. You need to research the pros and cons and make some educated decisions. Lots of factors play into this and lots of controversy surounds it. I am reticent to offer one best way of supplementing.
I would start with these 2 sites and look under articles that address nutrition and supplementation. http://www.chameleonnews.com/ http://www.adcham.com/