While you are right about the millions of possibilities and random results as I have said maximum concentration can be only to keep track of the number of spins without the bonus feature,which usually is around in 1000 spins it is a sure thing that you will hit the bonus round for example in the Play n Go slots provider that I play.This way if I have not gotten the bonus feature for 900 spins I will increase my bet as most probably in the next 100-300 spins I will receive it.That is the only concentration that you need in a slot game,except keeping track of the number of spins without bonus feature you cannot do anything about their programmed algorithm.
If something has a 1/1000 chance of happening and 900 times it hasn't happened, the probability of it happening doesn't change, it's still 1/1000.
For example, if you flip a coin now and it's tails, the next flip still has a 50/50 chance of being heads.