I propose the following:

1. Upon marriage, check who has the rarer surname
2. The spouse with the more common surname should change it to the rarer surname
3. In the case of divorce, the spouse with the changed surname should keep the surname

Of course, people are free to do whatever they want, but this is just an idea. Because unlike the convention of always taking the husband's surname — which inevitably leads to surname extinction and a thoroughly unequal distribution of surnames — this system would by my reckoning gradually lead to a completely equal distribution of surnames.