Hi Emkay
This link also might be a bit helpful - http://wikitravel.org/en/Nasik.
If you hire a professional taxi service from Mumbai airport to Nasik, you won’t face any inconvenience as mentioned by yogacombodia or suryadaya (I respect both of them for their genuine opinions).
I’m an Indian and my personal advice is: stay away from cheapest or cheaper transportation options if you can afford it. For example, you can travel from Mumbai to Nasik by paying US$ 1.5 to 2 and the same service can also be availed for approx. US$ 80 to 100. Here, first option is for financially weak people and second option is for lower middle class Indians. There are other options which are availed by middle class, upper middle class and upper class people and they simply don’t care about money and they also never face problems as mentioned by yogacambodia no matter how they look or what they wear. Here, I don’t mean to say that yogacombodia’s suggestions are wrong.
Personally, I feel if a person can pay approx. US$ 1000 for his/her air tickets, almost similar amount for his/her yoga course fee then why opt for US$ 2 or 3 bus services which are meant for poor people in India. Since these Indian bus travelers have little or no exposure to foreigners, they follow (not all of them) their typical instinct (mainly derived from western movies) as mentioned by yogacambodia. When I was a child (literate one) I used to think that when an Indian actor kissed an actress they kept a thin glass between their lips or face. You can imagine what our not so literate brethren might imagine…
Please remember - [B]Living in India is very expensive for most people and quite cheap for a few people.[/B]
These are my personal opinions (technically not perfect). If good, take it. If bad, dump it.
Enjoy, who cares. Spread happiness, yoga.