I suppose any tribute will make them less likely to attack you.

I suspect the following:

The AI is less likely to attack you if your relations are so-and-so or reliable and up from there. It may still attack you at so-and-so. This is logical.

If you are at so-and-so, some things may prompt an attack and some things make an attack less likely. I suspect that even if you pay 10 florins/turn it will help. I find that a bit against logic, the continued tribute makes a difference even if it is small, so it seems to have a separate additional influence from the relations with the other factions you see on the diplomacy screen.

Also, it is less likely for a catholic AI to attack you if you have high papal standing.

So, my tactic is to keep the relations at so-and-so with single gifts (maps/500 florins etc.) and also to keep up the small continued tribute. Also, I keep up relations with the pope.

I could be wrong, but so far it is working.