I've always been insecure of my smile because of gaps in my teeth so I avoided showing my teeth as a teenager. Im 18 now and had corrective dental work done so I pretty much have a perfect, white smile. I know this is a very powerful attraction tool and I want to maximize its advantage for not only pick-up but life in general (interviews, strangers, likeability, etc..)

Being so used to not smiling for all these years, how do I train myself in order to make smiling a second nature whenever I make eye contact with women or any stranger? I tried to smile at everyone I see which works for about a day or two, then I forget about doing it the next day.

Don't force yourself to smile, look at people in the eye, and when they look at you back, it's almost like a natural reaction to smile at them. Try this with every single person you walk past, do not worry about indicators of eye contact rejection.