How to keep site from being indexed whilst it's being built?

What is the most efficient way to keep the search engines from indexing my WordPress site while I’m building it?

I know I can check the Robots Meta NOINDEX checkbox on each page but that’s a pain – and then I have to remember to uncheck it once I’m finished.

Should I (and is it possible to) add a robots.txt file to the directory (folder) that WordPress is in?

In other words, my site is example.com/wp/ – can I put the robots.txt in the wp folder?

Thank you!