> Is it just a matter of excercising?
It certainly gets better with practice.
> Or are there other systems for estimation?
Apart from Planning Poker you could try "the estimation game", also called team sorting.
Another option (although controversial) is to assume one point per card (story counting), which encourages the authoring of smaller stories. I've had good results with this in certain cases. Martin Fowler had positive experiences with story counting as well, though only with advanced teams.
I wrote an article on these techniques: http://agile.dzone.com/articles/agi...n-practice