Do not daaven to change something that has already transpired (e.g. “my pregnant wife should give birth to a male”) – it is called a tefillas shav. (ShA1)
Within 40 days of conception, the embryo is not yet formed so it helps to pray that the baby should be a boy. After 40 days, daaven that the baby should be viable and be involved with mitzvos and good deeds. (MB1)
In general, when facing danger, daaven about safety for the future and express thanks when it is over. (ShA1)
Before counting your wealth, pray to Hashem to send blessing to increase the number. After counting, it is a tefillas shav. (ShA2)
Before a medical procedure or taking medicine say “Y’hee ratzon mil’fonecha Hashem Elokai, shey’hay aysek zeh li lirfuah, ki rofay cheenom atta”. Do not think your health can come through any way but Hashem, and put your trust in Him. (ShA4 – MB6)
When the procedure has had its desired effect, say “Boruch rofay cholim”. (ShA4)
If someone blesses you after sneezing, bless him back with “Boruch tih’yeh” – then recite the possuk “Lishuos’cha kivisi Hashem”. (MB7)
A person should regularly say “Everything that the Almerciful does is for the good”. (ShA5)