If you raised everyone's income above a minimum threshold, that would reduce income inequality by almost any measure.
Gini coefficient (the most commonly used inequality measure) effectively uses logarithms in its calculations: so raising incomes away from zero is the most effective way of reducing inequality by this metric (and most other metrics, and, I think, in most people's minds).
[Edit: are y'all down-voting because I said something technically wrong, or because you disagree in your mind? I'd like to know.]