Fenn traps, ferrets and .22 rifles are all things I've found to be useful, but ultimately you need to make sure that the hidey-holes are as few, and as uncomfortable, as possible. A good blast of strong air-freshener around know nest entrance points helps too: they really hate it, and seem to stay outside more (perhaps sizing up whether something has changed, which they are very wary of). This makes them more vulnerable to shooting and trapping.
Poison is a pain: it can work (farmers often use it - on the Isle of Man it used to be given out free rather than risk a total infestation). But the rats tend to die in inaccessible places, making terrible smells. Not so good for in or around a domestic dwelling...