The Laws of Prayer (The morning services) (33)

1. A person should try not to pray in a place where there is something which will disturb his concentration (e.g. there is a strong smell which confuses him etc.). Similarly, he should not pray at a time when it is difficult for him to concentrate (e.g. when he is not relaxed because of the weariness of a journey, or because he is angry etc.). (Shulchan Aruch and Mishne Berurah 98)

2. When a person is praying, he should think about the fact that no creature has the power to fulfil his request, nor does an angel, star or planet have this power. Everything depends only on the will of Hashem. (ibid)