Good vs Evil
Do you want to help people and preserve innocent life and are you willing to make personal sacrifices to do so? vs You're fine with not helping people in immediate need/danger and have no issues killing or harming(and putting people in a position to be killed or harmed) innocent people to get to your goals.
Addendum(Personal Sacrifice): Being good is not about being good when it is convenient for you to do so. Being good is about making real sacrifices whether it is putting your life, time, or gold on the line. A rich man tossing beggars a few silver is a good act...but not a GOOD act.
Law vs Chaos
Do you respect legitimate authority and the letter of the law? Are you more likely to call upon the authorities/seek official authorization or settle things yourself when the natural inclination is to get the authorities? Adventurers typically lean towards chaos or neutral on the Law/Chaos axis.
Addendum(Legitimate Authority): This is a tricky one. This is heavily influenced by the character's nationality, culture and religion. A tribesman is unlikely to take a town guard's word seriously, but would take a chieftain's word. If someone declared themselves a necromancer in Ustalav or Lastwall, you'd probably be killed on sight. A Milani or Caydenite is unlikely to respect any kind of authority in Cheliax. In other words, unless your character has a specific reason for WHY they do not respect the authority of the land, they should. Even if chaotic. Petty criminals are typically chaotic. Bureaucrats loopholing their crimes would be lawful.
Addendum(Oaths and Trustworthiness): A chaotic character can keep their word and be typically honorable without any issue on this axis. "Honor among thieves" is a common saying. A Lawful character has difficulty maintaining their lawful status without keeping their word.
Addendum(Free Will and Domination): How your character regards Indentured Servitude/Slavery to absolute freedom/anarchy falls under the Law/Chaos axis.
True Neutral: In my experience, it is very difficult to maintain a true neutral status throughout a campaign as it requires your character to lack any serious commitments to help others. To hook a character into a campaign your character has to feel someway about something and that often becomes a defining part of your character. You eventually risk your life to accomplish some goal and depending on how you go about doing so, you will eventually no longer be True Neutral.
Basically you have to be an animal or bug to stay true neutral.
The Friends and Family Clause: Ignore everything I said up above when it comes to someone your character deeply cares about. Mortals are irrational beings when it comes to their family and loved ones. Acts committed during brief moments of passion are typically not enough to change your alignment.
Once really isn't enough...unless it is: A single act or even a few are typically not enough to change a character's alignment. An honorable guard letting someone go in a moment of weakness or a good man killing someone in a rage isn't enough. A Good person betraying his party and allowing a necromancer to sacrifice a nation of people is a different story obviously.
Finally: Alignment is always a reflection of your actions, not the dictator of them.