There is any sure method, if you have a ranked account you can earn up to 0,1 btc each month (so far) writing in the forum and using signature campaign. But also it's not a safe method, they can change the payment rate, close their service, ban for spam etc etc etc...
However, signature campaigns should not be one's primary goal of being here. You should be promoting Bitcoin apart from the forum, helping newcomers and pushing towards global adoption. Like others have said, the amount of time spent posting just to get a minute amount of Bitcoin is an extreme waste of time, and you are much better off converting money earned at a job to Bitcoin instead.
Of course, absolutely I don't want promote signature campaign as a way to earn btc for "free".
Also there is an high risk to spam in the forum due low quality post and this is a big problem.
Just tell a way to earn some btc without using "fake" method like faucet, "investments" and so... and meanwhile help other users.
Normally I try to share info about btc with friends and coworker, an activity not remunerated
, but I hope also a global adoption of btc currency/technology related.