Although challenging at times to find sponsors, I was quickly able to bring in several hundred dollars per month in advertising revenue by directly approaching online companies who I considered good targets for my readership. I emailed them and asked if they would like to pay a monthly fee to place a banner on my site. Most said no, but some said yes and eventually I had a couple of loyal sponsors.
If you have a background in a specific field, you may find there is a demand for writing industry papers in your area of expertise. For example, there are often adverts for doctors, lawyers, nutritionists, and business experts from particular niches, to write white papers on different subject areas. These are well paid jobs that you will need to stake your reputation on, but that can often be worked on from the comfort of your own home.