I like using a jumpscript in a special directory which redirects to the sponsor. Then I put the directory in robots.txt as disallowed. This also gives you a way to redirect all links instantly from a sponsor if they decide to shave or shutdown.
If you think about it though the whole idea that we have to do anything is bullshit. We should not have to make our pages specially for search engines. It should be on Google to figure out what is a paid link. That answer about requiring machine readable text to identify the paid link just rubs me the wrong way. The next thing you know they will be declaring what is and is not valid html and refusing to index pages which don't follow their exact standards.