Usually anticipating your opponent's direction of movement is the best way to go.
Positioning is a huge part of playing Diana well because of how her Q works and her AoE E.
The fact that her Q is a crescent makes it a little more difficult to determine the precise end-point for the enemy, however when her Q comes out it's relatively easy to figure out it's range and avoid it by stepping into it if you're further away and away from it if you're closer, either of these will cause it to miss by a fair margin. Anticipating your opponent's movements based on their previous movement patterns will help you land Q more often and more reliably.