Rodents will not take bait unless the alternative food sources are removed. A slow acting bait in which the rodents do not realise what is killing them is most effective. Keep laying the bait until it is stopped being taken.
Baits must never be easily accessed by children, pets or wildlife. Make sure you follow safety instructions on the packet.
Do not poison in roof cavities as this encourages them to seek refuge in the roof and they may die there as a result. Warfarin based baits create high thirst in dying animals. They often move towards a water supply when dying (eg. a swimming pool).