I think the problem with a weak will emanates from the subconscious fear of failure. If one hopes to acquire a powerful will then they must do, I believe, two things.
The first is to establish what their will actually is. Too many of us don't look deep enough inside to discover what it is that we actually want for ourselves, and so what we will for ourselves or the world is constantly changing, making it impossible for us to ever realise it.
Once we know what it is that we are truly aiming for, the second step is simply to acquire the self-belief that it is attainable.
When we have both of these things - clarity of direction and belief in our ability to get there, working towards that end becomes far less of a struggle.