During my induction course for new staffs at Hosp AnNur, the motivator (Dr Azrul) told us all: two negative does not gives out a positive... our brain can't define it like that.
For example if you say " don't slam the door"; then your brain will break it to "don't" and "slam the door".. so unintentionally you are telling your brain: slam the door.
It is better to say "close the door softly"
He said that during his practical times, he used to pray for no accidents, no bad cases, no emergency operations.. and that is what that happens.. then he learn to change his prayer to the people be more careful when driving, everyone is safe and healthy.
So when you pray for something... say "I want something nice", rather than "I want something not bad"