There is a danger that personal principles become thought terminating cliches that act as excuses to do what you want to do. A good principle pushes you to do things that aren't always easy and a poor principle gives you an excuse to do as you please. For example, "All's fair in love and war", although perhaps funny and insightful, could be used as an excuse to treat others poorly. As personal principles are for your own use, it is fine if they should be cliche truisms. Cliches are cliches for a reason -- they are helpful and memorable such that people can't help but overuse them such that they start to feel obvious.
Actions have consequences.
Actions speak louder than words.
Ad astra per aspera – through difficulties to the stars.
Ad meliora -- towards better things.
After hardship comes happiness.
All beginnings are difficult.
All's well that ends well.
Amor vincit omnia – love conquers all.
An ounce of prevention is worth a pound of cure.
Appearances can be deceiving.
Aquila non captat muscas -- the eagle does not catch flies.
Be happy to hear criticism.
Be honest but kind.
Be humble in success and steadfast in adversity.
Better late than never.
Better safe than sorry.
Business as usual – continue normally in hard times.
Carpe diem – seize the day.
Character is destiny.
Do ut des - I give so that you may give.
Don't burn bridges.
Don't do the wrong thing just because it's a small thing.
Don't judge a book by its cover.
Don't look at a tree you can't climb.
Don't put all your eggs in one basket.
Don't put off until tomorrow what you can do today.
Don’t be quick to assume.
Don’t be quick to judge.
Dum spiro, spero -- while I breathe, I hope.
Eat to live, not live to eat.
Endure and the storm will pass.
Errare humanum est -- to err is human.
Every day is a second chance.
Every little bit helps.
Ex nihilo nihil fit -- nothing comes from nothing.
Face your fears.
Failure is not fatal.
Failure is the mother of success.
Fall down seven times, stand up eight.
Festina lente – slowly go fast.
Forgive and forget.
Fortuna caeca est – fortune is blind.
Fortuna favet fortibus – fortune favors the brave.
Give credit where credit is due.
Give more than you receive.
Good things come to those who work hard.
Happiness is not the absence of problems, it's dealing with problems well.
Humility is the root of all strength.
If at first, you don't succeed, try, try again.
If it ain't broke, don't fix it.
If you don’t stand for something you’ll fall for anything.
If you start something, finish it.
If you want something done right, do it yourself.
It is not what you know that matters, but what you do with what you know.
It’s not what you say but how you say it.
Keep it simple.
Labor omnia vincit – work conquers all.
Lead, follow or get out of the way.
Learn from mistakes.
Learn to walk before you can run.
Let bygones be bygones – don’t dwell in bitterness and regret.
Life is a journey not a destination.
Life is short.
Life is what you make it.
Listen more than you speak.
Live a century - learn a century.
Live and let live.
Luctor et emergo -- I struggle and emerge.
Measure seven times, cut once.
Memento mori -- remember that you will die.
Mind your own business.
Misfortune may be a blessing in disguise.
Necessity is the mother of invention.
Never interrupt someone who is making progress.
Never say never.
No pain, no gain.
Nothing ventured, nothing gained.
Nothing worth having comes easy.
Opportunities don't come twice.
Patience is a virtue.
Plan as if everything depends on you and trust in fate.
Practice makes perfect.
Pride comes before the fall - i.e. downfall.
Quality over quantity.
Qui non proficit, deficit – if you don’t advance you fall back.
Show respect to everyone regardless.
Sleep on it.
Speak softly and carry a big stick.
Step by step, inch by inch.
Strike while the iron is hot.
Success is not final.
Take the high road.
The golden rule – treat others as you want to be treated.
The heart is willing but the flesh is weak.
The more you know, the less you fear.
The most valuable advice is often unpleasant to hear.
There are no shortcuts.
There are two sides to every coin.
There is always a higher mountain.
There is always someone worse and better than you – avoid pride and envy.
Think of the consequences.
Think twice before you act.
Time heals all wounds.
Time opens doors for those who wait.
To be perfect is to change often.
To each his own.
Variety is the spice of life.
Veritas vos liberabit – the truth will set you free.
We are all in the same boat.
When in Rome, do as the Romans do.
Wisdom is knowing what you don’t know.
Work hard, play hard.
Work smarter not harder.
You are what you eat.
You can learn from anyone.
You can't please everyone.
You can’t buy time.
You only live once.
You snooze, you lose.
You win some, you lose some.