It is intellectually understood that the notion of prayer is only relevant to the future and not the past, for how could it have an effect on the past? Only thanksgiving is relevant to the past- to give praise to Him, may He be blessed, for the good that He did for him. Regarding the future, the opposite is the case- for praise is only relevant for that which already transpired, and prayer is relevant to the future for one is asking God to do something for him....

Therefore, one who enters a city and hears the sound of shouting due to some sort of calamity that occurred in it and says, "may it be [God's] will that [that shouting] is not from within my house", has uttered a vain prayer, for this prayer is regarding the past and whatever has happened has already happened. But he can say, "I trust that it is not from my house" if he is wholly righteous. This is akin to the story of Hillel the Elder, regarding whom it is said: He shall not be afraid of evil tidings; his heart is steadfast, trusting in the Lord (Berachot 60a). The matter can be explicated in two ways: (1) in its simple rendering- that he is not afraid that it was coming from his house or (2) because he had accustomed his household to accept everything with joy, both the good and its opposite. Therefore, even if, God forbid, some calamity had taken place, they would not scream, but would rather accept it with love and silence. ...

So too, if one's wife is pregnant and he wants a male child, he can prayer up until 40 days: 'May it be [God's] will that my wife will give birth to a son', since up until 40 days [the fetus] is merely water [viz. not formed]. But after 40 days, when the form has been solidified, praying 'May it be [God's] will that my wife will give birth to a son' would be a vain prayer, for what has happened has already happened, and it cannot be changed. Even though we find that Dinah switched from a male to female, this was within 40 days, and even if it were after 40 days, we do not mention [viz. draw conclusions from] miracles (ibid.), and the matters related to our holy forefathers were all miraculous.

One who enters a town says: 'may it be Your will, Hashem our God and God of our forefathers, that you allow me to enter this town in peace'; this is a prayer regarding the future. When he has entered in peace he says: 'thank You Hashem, my God, for allowing me to enter this town in peace'; this is thanksgiving for the past. So too when he is leaving, he says: 'may it be Your will..that you take me out of this town in peace'. When he has left, he says: 'thank You...for allowing me to leave this town in peace; just as you have allowed me to leave in peace, so too should You guide me in peace, etc.'. This is the wayfarer's prayer, as is explained in section 110, see there. Maimonides wrote in his commentary to the Mishnah that this is not a prayer or a blessing, rather it is merely a request. Therefore, we are no longer accustomed to saying this, since Rashi explained that [the reason it is said is] due to the criminal activity in the towns, and in our times this is no longer relevant. Nonetheless, it is proper to say for one who is careful regarding the words of the Sages, especially since this is not a blessing such that one would be concerned about a blessing in vain.

...One who goes in to measure his grain should say: 'may it be Your will, Hashem my God, that you send blessing for my stalks', since this is a blessing regarding the future. When he has begun measuring, he says: 'blessed is He who sends his blessing for my stalks', since he trusts that there will be blessing. One who has measured and then blesses has uttered a vain prayer, since blessing is only found for something that cannot be seen, so that it will not seem as though it is really going against nature, since most miracles are hidden ones. One who enters a bathhouse (their bathhouses had fires under them and were a constant danger) should say: 'may it be Your will, Hashem My God, that you allow me to enter in peace and leave in peace, and that you save me from this fire, and similarly in the future'. When he has exited peacefully, he should say: 'Thank You Hashem, My God, for saving me from this fire'. We are no longer accustomed to this, since the fire is now to the side and is not dangerous (Bach, Taz, Magen Avraham). This is explicit in the Palestinian Talmud in the 10th chapter of Berachot. One who lets blood should say: 'May it be Your will..that this matter will be advantageous to my health, since you are a free doctor'. After you let blood, say: 'Blessed is the healer of the sick'. Some say to say it with God's name and kingship (Taz, #3 and Maimonides, as well as the Beit Yosef in the name of the Semag). It is proper to say before every healing: 'May it be Your will, Hashem My God, that this will be healing for me'. This is how the punctilious act. One should accustom himself to say: 'All that the Merciful One does is for good'. The principle is that one should always prayer for the future and beseech for mercy before Him, may He be blessed, and he should give thanksgiving for the past, thanking and praising according to his capacity. The more praise one accords to God, the better. This is like it says in the verse: 'it is good to praise God, and to sing your exalted name'.