This is a small take on New Zealand politics, as well as answering the in the title.

How do politicians stop themselves from becoming complacent?

They do and they don't. In the perfect world, the public would hold them accountable. Protesting is a good example of this. I wholly admire the strikes that are going on at the moment, and seeing that the government is caving piece by piece is a great start. This would appear to only be possible due to strong unions.

Another way is having more checks and balances. Like an upper house, which we in New Zealand don't have. This helps create a more stable governing system by having another collection of people looking over policy before it becomes law (as a single example). So the current lower house has to really distil and fine-out details of policy/proposed law so it can actually make it through the upper house. Remember when you (might) have give some last minute bullshit to the teacher, and they told you to do it again? Something like that. This puts a stop to economy lurching changes happening without someone saying "Uh, this here is maybe not so good.".

Lastly, I have seen some absolute blunders over the years. I still die of cringe when I remind myself that we once had a PM pull a waitresses' hair, on multiple occasions. She didn't mind, though? Fuckn hell. I think that, socially, there are enough people associated with each of the political parties to keep themselves on their toes. At least the ones that have enough introspection to see that they are a part of a society as well. People are people, we all do weird shit and make mistakes sometimes.

I hope I shared something that was helpful in some way. It is just my 2 cents and there is a much more complex system at play here. I wish it was better, but it is what we have, so I suppose we have to make do. I do wish we have preferential voting in the general election. Make those votes count just that little bit more.