Most blogs have a sidebar that replicates across each of the pages on the site. This method of monetization involves selling a link within your blog sidebar. This is usually on a monthly or yearly basis.

Advantages: This literally takes a couple of minutes to implement and you can have multiple links running across your sidebar/blogroll at one time, so the revenue potential is pretty good, especially if you have an authority website.

Disadvantages: Google have been cracking down on sites that offer this kind of thing.

I personally know of a lot of travel blogs that were hit during the Penguin updates for this kind of tactic, so you really need to evaluate the risk vs return.