The trick is to decide with disk you want to move to which post. Calling the posts A, B, and C left to right. What you are really trying to do is move everything off the bottom disk, then move the bottom disk and move everything back onto it. So to move 7 disks, you move 6 disks, the last disk and then those same 6 disks again.

A trick if you lose track of where to move what:

Select the disk you wish to move (say from B to C). If there are an odd number of disks above it, move the top disk away from the target pole, in this case A. If there are an even number of disks above the disk you’re trying to move, start by moving the top disk TO to target pole, C.

You can really see how this happens if you start with 3 or 4 disks.