5. Relevant Advertisers
Another way to make some money with your site is by contacting relevant companies and ask if they would like to advertise on your site. If this is the direction you want to take I would recommend you have some traffic and stats to show your potential advertisers when you contact them.
A plugin that makes all this very easy that I use myself is OIO publisher, it automates the whole process. You can add the ad blocks where you want them, the advertiser clicks on them and can sign up right there and then, payment can be done through Paypal.
If you want to go the easy way you can go through a company like buysellads.com that will connect the two, website owner and advertiser.