At times we have certain files which grow mammoth in size with time, like with RoR applications, there are log files, residing in the log directory.
Something like:

We can clear up the files using the below command in a single line.

for i in *.log; do cat /dev/null > $i; done

And voila,

Of-course you should have a backup copy of these logs in S3 or logstash or somewhere, and I am assuming you have provisioned for the same. I am writing up a short post on how I have done that, you can follow that incase you haven't put those services in place already.