The Decisive Moment is a term coined by famous street photographer Henri Cartier Bresson. Basically he felt that the moment is fleeting, meaning that once you miss it, its gone forever. Understand that he was photographing things like people jumping over puddles.
How is this related to crossdressing? The decisive moment in this case is much more complicated. Your priority is still very much about timing, but timing for you probably means is there enough time to do your makeup, paint your nails, dress up, remove your makeup and nail polish, and then put everything away. And what about perfume? You dab on a bit of Daisy by Marc Jacobs and now your bedroom smells like a flower shop! Which is not a bad thing, but you might be expecting your friends later for poke her (poker).
Yes timing is everything and the moment is always fleeting, but most guys I know have their dress up sessions down to a science. The key is to relax and enjoy it even if you only have an hour or two.
As for a quick crossdressing selfie, take an extra minute to set it up. Experiment with different poses and backgrounds. After all, you’re putting in a lot of effort, and when you look at your photo collection in your seventies you’ll wish you took more.