Ask HN: What techniques do you use to validate your startup idea?

I've been searching for ways on validating startup ideas - and one single advice that I found reaper over and over was building a landing page, explain the project (maybe even add some pricing info) and try to collect as many emails as possible.

I've done something similar with my new project - http://www.pinmark.in - and I've managed to gather about 25+ email addresses but I don't feel like the this is real validation and that there's still a big enough market to focus my time 100% on it.

What other techniques you use to validate your startup idea and when do you have enough validation to commit and start working on it?

P.S. If you have any feedback on Pinmark.in please feel free to drop a comment. Thanks!