Generally: don't trust anything from the outside world or anything that can transit untrusted infrastructure, that means check types and sanitize values before passing along. Break loudly and quickly to get attention for a fix. Keep the codebase as tiny as possible too.

Ruby: recompile with minimized OpenSSL 1.0.1+ (LibreSSL when possible) and with patches that improve Ruby's default OpenSSL security.

https://gist.github.com/steakknife/8228264

https://gist.github.com/steakknife/10092587

https://gist.github.com/steakknife/10096008

For Rails apps: use brakeman as one part of security audit strategy

For gem authors, sign them (please!): I wrote waxseal to make it dead simple

    [sudo] gem cert --add <(curl -L https://gist.github.com/steakknife/5333881/raw/gem-public_cert.pem) # adds my cert (do once)
    [sudo] gem install waxseal --trust-policy HighSecurity

For gem users, find which aren't signed

    Add this to ~/.gemrc gem line:

    --trust-policy MediumSecurity

    or just if there's no gem: .... already: 

    gem: --trust-policy MediumSecurity

For anyone using git, sign your tags (git tag -s ...) and commits (git commit -S ...) por favor