Motivation:
Increase donations to better improve overall gaming expeirence.
Recommendation:
For those who donate, a kind of reward can be given to them which exemplifies them as a donar in game. Maybe this is a rank symbol next to their name or a special skin mod for their units. Some of these rewards could be shown in game and also in chat/forums. As a side effect, this would increase awareness of the community regarding those people who donate perpetuating the cycle of donations ultimately making the game better
Downside:
Initial overhead for writing the code to enable rewards etc.
Initial overhead for marketing the rewards for donations.
You would have to ensure that past donors would get the rewards too.