The same way we do now? There's a strong cultural expectation that one will agree to review, or have one's graduate students review, a reasonable number of papers. If you're a member of the Foo committee of the Society for Bar, you're not going to refuse to pull your weight as a reviewer for the Society for Bar's International Journal of Foo unless you want to get the cold shoulder at your next conference. It's not like anybody gets paid for this today.