The way to do it is to give a very generous salary and to simultaneously crack down hard on even the appearance of impropriety. Neither is being done right now, and part of the reason is that you can't really do one without the other[1], so you're stuck in a bit of a catch-22.
[1] You can't crack down on on public servants questionably but not clearly criminally making money on the side if everyone that's supposed to be doing the cracking down are also public servants who sympathize (more cynically, are in on the same gig) because they're also making a fraction of what their peers are making. And you can't pay your public servants more when the people are fuming at the individually arguable but clear-in-aggregate corruption that is rampant.
[1] You can't crack down on on public servants questionably but not clearly criminally making money on the side if everyone that's supposed to be doing the cracking down are also public servants who sympathize (more cynically, are in on the same gig) because they're also making a fraction of what their peers are making. And you can't pay your public servants more when the people are fuming at the individually arguable but clear-in-aggregate corruption that is rampant.