I don't have the most consistent or best ollies, but I can pop some good ones on occasion. What I have found helps:
A.) Making a pretty exaggerated circle with the while compressing the legs, first over the front foot, moving weight to the back foot, and then exploding out off the back foot. When I'm not popping good ollies, I tend to not load enough on the front foot and just snap off the back foot. Works in some situations, but you definitely don't get the same power.
B.) Keep your upper body mostly upright, with hips and center of gravity over the board. What I notice happens when I pop shitty ollies is I break at the waist and lose a ton of power that would otherwise be going into the board.
C.) Keep the upper body quiet and your shoulders parallel to the direction you're trying to go. On bad ollies, I let my upper body get wild, open the shoulders, which opens the hips, and throws me completely off balance in the air.