How to ferment dosa batter? I have never been successful in fermenting dosa batter properly in the US. I generally soak urad dal, rice and fenugreek seeds for a few hours and grind them together. Even if I leave the batter in 30°C temperature, the batter does not double in volume, as is generally does in India. The batter seems to ferment somewhat because of the sour taste but it seems that the fermentation is not being done by the "right" microorganisms.

Any ideas on improving fermentation, or on mimicking the process they use in India?
I found some pointers here: 
http://www.indiacurry.com/south/batterexplained.htm

(The following is just taken from the information in that link, I do not actually know anything about dosa)

It seems that the lack of fermentation could be due to a number of things:  


Overwashing the ingredients (removes the wild yeast).
Using chlorinated water (kills the wild yeast). Use bottled water instead of tap water.
Iodine in the salt can also kill the wild yeast. Use kosher salt instead. 
Temperature. As you have already noted in your question, normal room temperature in the US may be too low. You could try putting it in the oven with just the light on, no heat. Or if you have a gas oven, with just the pilot light on.