Always carry spare inner tubes and basic tools/pump.
Always stop to offer help to a rider that has failed to do the above.
Always carry a water bottle and a snack, usually a flapjack.
Make eye contact with drivers where necessary/possible -NEVER assume they've seen you
Always give a nod of approval or finger wave or audible thank you if a driver or another rider lets you in/out, or is gracious to you in another way like gives you a nice wide space when passing. There's enough cycling nobbers out there giving us a bad name, and cycling haters in general, so I like to try and convey happiness, thanks and positivity in my cycle experiences.
Don't ride when there's ice on the road, just not worth it. I've been downed enough times now to know better.
Don't ride in winds over 40 mph, again just not worth it, getting a surprise side gust cycling by the sky scrapers in a city centre can easily blow you into the path of oncoming traffic.
Regularly check tyre pressures and tyres themselves to remove any tiny flints that are in the tyre and can turn in a puncture any minute now.....
Life is to short to skimp on cheap/rubbish tyres, treat you and your bike, you're worth it and they are better.
If someone pulls a dick move, let it go, if you got away unscathed no harm is done, no point stressing over it, besides another nobber will be along any second so don't raise your blood pressure unnecessarily.
And the final one, not a rule as such but a happy side effect of cycling, wear a smile. The day will go better.