I don't know how Yahoo went about it but Microsoft for example gave plenty of notice and announced that accounts that haven't been touched for X years will be recycled. When I'll make this move (soon) I will definitely squat the account even if I just end up scheduling sending a mail to myself once every X days.

In parallel I would keep an eye out for policy changes. Like when Google will inevitably tell me that in order for the service to stay free I have to provide a minimum amount of private data per month :D.