Language is the very first step I've noticed. I'm a cisgendered female but I have multiple friends who don't identify as male/female and usually prefer if I use 'they' rather than him/her.

I've noticed that, while I still slip sometimes, I've started looking at everyone around me (that I don't already know for sure), as 'they' and gender doesn't really matter from there. Rather I'm quicker to notice if someone has nice eyes, or a neat name (which I might see on someone's ID or receipt at my old job), or something similar, rather than gender.