^Without getting into spoilers? By gathering as many war assets as possible through scanning and exploration and by making the decisions most likely to enhance your chances, both on a micro and a macro scale. Seriously, even something as seemingly innocuous as settling a bureaucratic dispute or resisting the urge to knock someone out can have an impact. Some avenues will of course either be open or closed to you depending on decisions made in previous games (assuming you import.)
I got upto just under 3800 (effective) and that was without fully completing several rewarding sidequests either due to bugs or not turning them in before a particular point in the story. Had I bothered to reload an earlier save when I realized something was off, I would have made 4000 easy.