Multiple Goons >>>>>>>>>>> Lone Goons.
If you play with Travellers, cycle like your life depends on it.
Look at ALL the cards when you start. ESPECIALLY those that aren't in the supply, or even technically "cards."
In real life, keep track of points. (I don't follow my own advice here, but still worth doing).
Thin before you green. (I didn't think this needed to be said, but I can never forget the poor soul who opened Duchy on a 5/2 and bought Chapels in bulk when the Provinces were low.)
And the most important advice for any board game: Have Fun!!!