I've been exchanging a few messages with Daniel of Kirtan Central via the mridanga.net facebook page. It's really good to be in touch with musicians and teachers across the world.
Daniel runs his own blog here where you can find out about all of the great work he is doing teaching harmonium / kirtan around California, Massachusetts, New York and beyond. On the Kirtan Central site, you can also buy kirtan instruments.
It was nice to see in his press photos that Daniel is using a BINA harmonium. I visited their Southall store yesterday. He asked about tuning the baya. I forwarded him the following (full size version here) mridanga illustration to help with my description.
Jas Musicals in Southall recommend applying a small amount of water to the madian. Try not to get any on the syahi as this will dissolve and erode it (some damp cotton will help you apply it with accuracy). The water encourages the skin to stretch / loosen. You may need to do this a few times (let it dry in between applications) to get the slack you are looking for. Don't get the kinar wet as it will start to warp and raise from the skin beneath (possibly causing a buzzing sound every time you hit the drum).
BINA Musicals, also in Southall simply suggest to store your drum baya side down on a cold surface. I think this works in the same way by attracting moisture from the environment. Do keep checking your drum though as it could become too lose. They also said just to keep playing as this will loosen the skin over time.
When I visited BINA to buy my first mridanga in 1993, Manu.S.Sura said to care for my drum as if it were a baby. No sudden changes or extremes in environment. If you find that your drum skin has become loose (this is more common on the baya side), then you may have to find somewhere warmer and drier to store your drum. Changing how you store your drum will usually do the trick, but be careful as repeated stretching and shrinking of the skin will inevitably cause more permanent damage.
All of this is not so important with the fibreglass Balaram and Tilak mridangas as the mylar used for their skins is not hygroscopic.