The first and foremost weapon against tyranny is the vote.
The knowledge that abusing power means losing power at the next election will discourage most wannabe tyrants.
But more is needed for this to work.
Respect for the law
If a political leader says "Screw democracy, I'm not stepping down." the people around them must say "No. I'm not obeying you after your term is over." They must be confident that almost everybody else will say the same thing.
If a military leader says "Screw democracy, I control the guns.", their subordinates must refuse their orders. They must be able to trust that the only gun the leader really controls is their own sidearm.
But respect for the law is important in another context. If a politician is caught breaking the law, the voters must stop voting for them. If a political party has several bad apples, stop voting for that party. This is the only way to keep them honest. (Note to Americans: You have more than two parties. Just saying)
If voters let politicians get away with minor crimes, they will commit greater ones.
Freedom of the press
If a leader does something the voters would not like, it is important that the voters learn about it.
This is the mission of the Media. Exposing the misdeeds of the powerful is their contribution to keeping the system working.
Laws like the Freedom of Information Act are important. Government secrets are BAD for democracy. Whenever somebody talks about "National Security" there is a very good chance they are actually covering up something the voters would not like.
Media these days means more than it used to. There is still TV and newspapers, but in addition individuals can make contributions through blogs, vlogs or similar.
Respect for the truth
This section is the weakest link in most democracies.
There is an awful lot of lies around. Politicians lie. Media lie. It can be very hard to sort out the truth in all this noise.
Most importantly, people must stop believing everything they hear. Just because the politician you already like says something, doesn't mean that it is true.
Check your facts. This can be hard, but if you aren't certain, at least don't repeat rumors.
Then people must use their main weapon, the vote. If politicians lie, stop voting for them. If media lies, stop watching/reading them.
As I said, this is where most democracies struggle the most. Please do your part to help your country. Vote well.
Separation of powers
Americans are fond of pointing out how separation of powers protects them against tyranny. There is some truth to that. During the Trump term, we saw several times how the other powers stepped up and stopped him one way or another. Good work! I gained a lot of respect for SOP in those years.
Nevertheless, I would like to claim that if the system needs SOP to be stable, it is too fragile in the first place. It is a good last resort, but it shouldn't be necessary.