I should have posted this earlier, but here goes anyway:
Zorb-it is having a 50% sale on their online store - it's closing sale, so you can stock up. Valid till 30th June.
Add the code ZORBIT50 while checking out. Shipping is a little steep but for those with pricey instruments, this is a pretty valuable piece in your kit.
Basically Zorb-it maintains the humidity in your guitar case at 58%, right at the optimum point for your guitar. Given how badly the temperature and humidity fluctuate in our country, those with solid-topped and all-wood guitars will benefit the most. Hard case users will see maximum gains, though even a gig bag will be quite grateful. One packet should last 5 years easily, though given the severity of our conditions two packs to a case is about right. A set of four should last ten years easy (I'm on my seventh year with the first four and they still work great) - though they will start feeling squishy in the monsoon and hard and dry in the winters, they still do their job fine. Can be dried up or moistened with a few packs of silica gel, or a damp sponge kept with them in a sealed pack.
Guitars swell during the monsoon if the humidity is not controlled, and they shrink in winters when the humidity is low (<30%). Good instruments tend to suffer more than plywood ones, and acoustics are more severely affected than electrics. Our bigger problem is the combination of high humidity and temperatures, which cause solid woods to soak up water form the air like a tree would, and warp necks like twigs. I've not had a neck warp on me since I've used it - and Calcutta has 90% humidity for five months a year.