I'd suggest max Hp at first level (to avoid having characters who fall unconscious at the drop of a hat).
For later levels I'd suggest I'd suggest rolling you full HD and using the new number if it is higher than your current hit points. e.g. a third level barbarian would roll 3d12, if that roll is higher than their total hit points then that is their new total.
It preserves some randomness while producing characters who overtime will trend towards the maximum hit points for their class.
