Free directory submission is the cheapest and easiest way to build backlinks and improve your serps. Even today you can beat many competitors with this method but it should of good quality. Free one way links are more important than link exchange.
Few points that you should look for, before submitting to free directories are listed below:
:- Don't submite your site to directories that have PR-N/A.
:- Avoide submissions to directories those pages are not even indexed.
:- For quick approval try to submite your site to newly launched directories.
:- Submite in directories according to submission's rules.
:- Please don't use an automated submissions services.
