I think it's more of a science but there are gaps in my method. I usually stay dry in all-day rain including occasional toad-stranglers. Multiple days of rain and high humidity are a problem.

I usually wear a jacket that overlaps the pant crotch so I don't carry around a puddle. The Aerostich Darien pants have a very vulnerable simple zipper fly but the Darien jacket is long enough to keep water off.

I added a rain gutter to the main zip on a Tourmaster Advanced jacket. It would keep out rain for a half-hour or so, but driving rain would eventually force through the zip and soak me. With the folded gutter it worked great in the rain.

My weak spot is gloves. Most of my MC handlebars are low and I tuck glove cuffs into my jacket so water running down my sleeve continues outside instead of into the glove. This works as long as I'm riding, but taking the gloves off for fuel, bathroom, changing maps in the case, or lunch will get a little moisture in so eventually I have gloves with a wet liner. As long as I can keep gloves on I'm OK. For cold rain, the handlebar muffs (like Hippo Hands) keep my gloves and sleeves drier.

I keep hearing that liners like Gore-Tex will get holes and fail, but I haven't yet had that happen. When I get wet it's because I allowed a gap or I had to use my bare hands too much.

I've had same problems. Gloves are a tough one. Plumbers rubber gloves over
riding gloves can work ... but not a good feel. Surgical gloves UNDER riding gloves adds warmth ... but gloves still get wet. Tough one. Some gloves pull the liner inside out when you remove your hands ... then you can't get your damp hands back in the gloves!
I hate that!

Held one piece rain suit plus army boot and glove liners work. You need to dress for it though which means either changing at the road side or going boil in the bag if it stays nice.

If you want to wear the same kit wet and dry, heavy waxed and fully treated cotton works. Stinks in a tent though.

I am yet to find Goretex kit that works when three years old, dirty and there is serious rain.

Andy