Restating the basics. To make money on a site you need two things: Traffic and a way to convert that traffic into money.
You have a site. You have ads on that site. You have content and good keywords on that site that should cause search engines to serve links to your site.
Now what you need to do is convince the search engines to place you on the first page for relevant searches.
Getting links to your site helps convince the search engines to place you higher up the list. Directory submission is one way to increase that placement (It will also bring a little extra traffic).
Submit your site to every relevant free directory you can find. Start with the directories listed here
Namepros When you've worked your way through that list move onto
dmoz and don't forget to submit to DMOZ itself, they may take forever to list you, but it costs nothing to submit & you might get lucky.
Be a little cautious about back-links to the directories. Kiore's first law of links is "Every link from your page must be there for the purpose of making you money" In this case you need to believe that you will lose less revenue producing traffic through the link than it will bring in. If you decide you want the back link, first check that they are in google before linking to them, there's penalties for linking to sites google has banned. Once you are sure they are OK, if they require a reciprocal link & you want to link to them, go ahead.
