Here is a pretty cool tool which may save you from some robots.txt errors. Now, writing a robots.txt file is not the most difficult thing in the world, but it never hurts to check.
All you have to do it create your robots.txt file, upload it to your web server and then paste the URL (usually, http://yoursite.com/robots.txt) on Motoricerca’s web site and you will be alerted to any potential errors in your file and you will be shown how to fix them.