You will get a PU if the ruler dies without an heir with a good claim, but that happens far less then them having a weak succession at some point at all, which enables you to claim their throne.
So, if you really want that PU, War is the best solution. If you want a peaceful PU, you can just claim thrones and hope for the best, but be careful, because you will lose a lot of relations with everyone you have married. Also, if they have an heir with a weak claim instead of no heir, your chances improve vastly, because in this case for you not to get the PU the heir needs to die before the king, and then an other heir with a better claim needs to appear, which obviously is far less likely then just the appearence of the heir before the king dies.