There are two primary factors involved in throwing an axe well: consistency of throwing motion and distance from the target. The most difficult of these is consistency. The idea is that for every throw you swing your arm at the same speed, with the same force, and release the axe at the same point in your arm rotation. I've heard it said that it takes 3,000 throws to develop a consistent throw and 10,000 for it to fully develop into muscle memory.
The distance you need to stand from the target is based on your throwing motion which is why consistency is important. Standing to close to the target often results in the axe handle striking the target (under rotation). Too far away and the top of the axe head will hit first with the handle parallel to the ground (over rotation). In the ideal throw, the axe should stick into the target at a slight angle with the end of the handle slightly further from the face of the target then where it meets the axe head. It is often helpful to have another person watch your throw to determine whether you need to move closer to, or away from, the target.
I suggest that new throwers stand with their off foot forward: a right handed thrower would stand with the left foot forward, a left handed thrower with right foot forward. Another simple tip, as you draw your arm back to prepare to throw, make sure the cutting edge of the axe always faces toward the target. This prevents in-flight wobble and allows the cutting edge to strike cleanly which means it will stick better into the target.