I have several:
Music theory matters and learning everything about it should be one of your top priorities
Just because a few people out there have different tastes/opinions doesn't mean it's all "subjective"
Get it right at the source; "mixing" can't do anything about bad pieces and crap timbres
It's better to be great at one genre of music than middling in many
Don't forget about woodwinds
Avoiding "parallel harmony" in melodic lines to mentally appease the spirit of Bach, who is dead, will hold your orchestrations back from their full potential
Work with live musicians as much as possible
Compose something every day
A 2 minute piece people want to play on repeat is better than a 5-minute epic they'll only listen to occasionally
Improvisation has its place, but you should use it sparingly; every note should have real thought behind it.
Friends don't let friends imitate Schoenberg
A recording/mockup should be seen as inseparable from composition rather an interpretation of it and should therefore be as polished and the very best presentation of your piece as possible; "raw" or "live sound" is usually a cope.