I agree with all of the above, but the real trick to making a feature film on practically no budget, is to have a fantastic script that people will want to work on and to surround yourself with highly skilled, talented and creative people, who will love the project so much that they will devote their time and skills to creating it.
The people you work with need to feel that it is as much their film as yours, that they have as much to gain from doing it right as you do. Low budget indie film making is not the home of autuer directors, it has to be a creative collaboration.
If you create the right creative atmosphere and refuse to compromise on the production values, if the people that you choose to work with have access to the right resources, then your budget can come down to tape/film stock, insurance, transport, catering, a small props budget and a contingency fund to solve any last minute problems.
At this point, there is no limit to how low your budget can become.
Everything else is just about good planing and patience