In three ways:

1. use a more number of traffic generating methods including but not limited to online advertising.

2.build priceless content on their sites which attracts AdSense ads which pay out the most when they get clicked.

3. use copy on their websites that encourage clicks on ads. Note that Google prohibits people from using phrases like "Click on my AdSense ads" to increase click rates. Phrases accepted are "Sponsored Links" and "Advertisements"