A MtF in my area who is just starting her transition asked me advice about passing. I want to post here what I told her, in case anyone finds it helpful.
Imagine there is a boulder just at the edge of a hillside. Imagine you don't want it at the bottom of the hill where it might damage something. Your very best hope is that it stays put. Once it starts rolling downhill, your chances of stopping it are not very good.

Before I go any further, I should mention that I don't pass perfectly. I pass often enough that I'm pretty surprised when someone clocks me, but I do get clocked, especially in LGBT spaces. In particular:
* I'm 5'11" tall.
* My jaw is on the wide side for a woman.
* My hips are narrow, much narrower than my shoulders.
* I have size 12 feet.
* I wear a wig nearly all the time.
* I wear large glasses frames to cover my eyebrow ridge.
* I've never had FFS or a trach shave, so my Adam's Apple is front and center for everyone to see.
None of these things will get me clocked. All of these things are unobtrusive and might be found on a cis-woman. Nobody will see any of them and think I'm trans.
But taken in their totality, someone observing all of them could easily conclude that I'm trans. In other words, if someone has suspicions and begins looking carefully at me, there is no way for me to hide my history.
IF they are suspicious.
So for me, being clocked is like a boulder rolling downhill. As long as I don't give people a reason for being suspicious, it is unlikely they'll put all the indications together and clock me. However, once something stirs their suspicions - a tuft of arm hair, face stubble, my craggy forehead with its unmistakably male aging, my baldness pattern, a drop in my voice - the boulder starts rolling. Once they ASK themselves the question, "is this person trans?", I've already basically lost the battle. Nothing I'm going to do to myself will stop the boulder before it gets to the bottom of the hill.
So for me, passing is making sure they don't have a suspicion. Wherever they look first, my feet, my legs, my trunk, my head, my arms, my hands, they have to see girl. If they see my ears first, they should see feminine earrings. If they seem my face first, my wig should show them a feminine hairstyle. If the seem my arms first, they should be clean shaven with bangles or bracelets. Etc.
Cis people have trouble with this concept. My electrologist keeps telling me "just because you have stubble [on the days before my appointment] doesn't mean people will know you are trans. Lot's of women have stubble." Friends have told me, "you don't need to hide your forehead and eyebrow ridge. Lots of women are balding and have prominent eyebrows."
But those lots of cis women with stubble and prominent eyebrows are not trying to keep a boulder at the top of the hill.
I hope this helps.