how do i get people to understand the fe

I am a it consultant and and my main focus is on providing project consultancies to companies.Most of the companies specially so in india just want to jump into the IT cause theres a lot of money in it. In doing so they
do not realise the kind of work involved and the investment required.That to some extent is bearable..what is worse is that they come up with some fancy imaginative ideas withought any base and expect to get outputs withought understanding the feasibility of such projects.Some of them just drive you up the wall with their time frame and their demands.You cannot risk telling them that its not possible as it is reflected upon as your incompetency…How do you tackle such people??