1. Addition of Fenugreek Seeds (Methi)
Though not really necessary, adding Fenugreek seeds helps to aid fermentation. So add in about 1.5 tsp of Methi seeds (my ratio of rice and urad is 3:1).

2. Grinding Separately
The Rice and Dal has to be ground separately. This results in increase in volume and also aids in fermentation.

3. Using Filtered Water
Use non-chlorinated and/or filtered water since chlorine inhibits growth of wild yeast which is essential for fermentation.

4. Using Soaking water to Grind
Though I have no scientific reason to back this theory, from my personal experience, I find that using soaking water (in which you soak your urad dal and rice) to grind the batter also helps to aid fermentation.

5. Adding enough Salt
At times, not adding enough salt to the batter after its ground can also be a reason for lack of fermentation. Try to add at least 1/2 tsp (my mother mother suggests 3/4 tsp- 1 tsp) for every 1 cup of dry ingredients (rice +urad dal). You might probably need to adjust later according to your taste.

UPDATE: Though I have heard many say this as a point, I personally find NOT adding salt after grinding aids better fermentation since salt is actually an yeast inhibitor. I add salt only after it ferments. If you have tried other tips without success, then give this one a try.

6. Avoid adding ferment inhibitors
Avoid adding ingredients like Baking soda, Baking powder and Yogurt before the batter ferments since these inhibit the growth of wild yeast. If you really have to, add them after the process of fermentation.

7. Consistency of the batter
Adding too much water or too little will hinder fermentation. Give the batter a good stir with your hands instead of a ladle. The warmth from our body will help to kick start the fermenting process. Your end batter should have the consistency of ketchup, flowey, smooth and velvety in texture.

8. Fermentation Temperature
Ideal temperature for fermentation is 80-90F. If you live in a warmer place, then your fermentation should take place around 8 hour mark.

If you live in a place with cold climate you would have to take the following precaution:

• Place your vessel with the batter in a warm, draft free place. Mostly this should be in a pre heated oven (switch off the oven though). Leave the Oven light on since it will give enough heat to keep the temperature steady throughout the fermentation period (mostly overnight). This should take about 10-12 hours.
  
• You can also wrap a shawl around the vessel to keep it warmer. Make sure to use a huge vessel (which would accommodate even the doubled up volume of the batter) in that case. If fermentation occurs, your batter might overflow spoiling your shawl.
  
• At times, if you use a room heater, placing the vessel in that room would help in fermentation.
  
• I saw an idea where someone used a heating pad to proof their bread. I assume, the same method might also help in fermentation as well.

9. Avoid over sanitizing the environment
Over Sanitizing our houses (air filters etc) along with closed windows throughout the day, inhibit the growth of wild yeast in the batter leading to lack of fermentation.

10. Avoid using Air tight lid while fermenting
Do not close your vessel with the batter with an air tight lid. Instead simply loosely cover it with a plate or a cloth.

Last Resort (if above doesn't work)

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There are three things you can do in such a case.
- Use a different Rice and Urad dal. They might be contaminated or old. Also try to opt for Whole Skinned Urad (namely Urad Gota) instead of split.
- Try a different recipe.
- Use yeast. I recall noting this info from a website (source forgotten, will update later) where they suggest using half tsp of yeast in half cup water and a teaspoon of sugar and then adding it to a gallon of batter.

Check out classic South Indian Breakfast Recipes

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Idli	Kushboo Idli	Sabudana Idli	Coconut Chutney	Sambar
Sada Dosa	Thalippu Dosa	Sprouted Ragi Dosa	Karuppu Ulundhu Dosa	Vendakkai (Okra) Dosa
Egg (Quinoa) Dosa	Sprouted Mung Dosa	Carrot Oats Dosa	Rava Upma	Ven Pongal