I live in a very dry location and hadn't ridden in rain in YEARS....until 2 weeks ago. Took a 'work' trip and left in 43 degree rain. Rode in it for 90 miles before breaking into the clear. My advice, rules, whatever you want to call them:
If you know its going to be a decent rain, wear a rain suit over your regular riding gear. It doesn't have to be a $500 Klim rain suit. My $60 Tourmaster works just fine and when the rain stops, I take it off and I'm dry.
Someone said it before....shiny pavement is slick(er) pavement. Forewarned is forearmed.
Close the top vents to your helmet. Cold water on the scalp doesn't help matters.
Keep the chin vent open. It helps defog your face shield.
Make sure you have decent tread on your tires. Doesn't have to be new tires, but you don't want to be down to the wear bars. Modern tires sipe water incredibly well, and that's your traction.
Biggest thing (and hardest to do). Relax. You will stiffen up and be tense..arms, legs, head torso, and that will transmit throgh the bike and make your handling even more sketchy. Loosen up. Be free. Don't tighten up in the corners. Trust that your tires and going to do their job (they will barring panic situations), relax and you will end up having a fun time riding in the rain!
Remember the line from the Billy Joel song in the late '70's....'Even rode my motorcycle in the rain!'