On my site, I currently have no special rules for search engines. It is a blog, statically generated using a Python program. When I search for some of my articles on Google, there is usually a tag or category page included in the results. Sometimes it even ranks ahead of the article itself. Obviously, as these links aren't always going to have the article on them, this aren't the results I want people to click on.
So, I'm thinking of setting noindex on these pages. Is there any possible downside to doing so? Is this possible to do via robots.txt, or do I have to add it to all the relevant templates? All I can find for robots.txt are ways to stop the search engine crawling those pages, which isn't what I want - while I don't want them indexed, it's still the only surefire way to find all my blog posts.