The basic idea is to use A/B testing. And what is being tested is not just the wording of the messages asking for donations but also what rewards will be offered (for example prioritized support, voting on features, a thank you in blog posts. see here for a list of rewards).
To prevent someone from starting to advertized the rewards that are just for him as part of the experiment. A code could be issued for each user that can only be used once to get the ability to pay for the rewards.