You can save some gold by moving your army slider to the left. This decreases army maintenance significantly, which is important if you need/have a large standing army. Your armies will have very low morale from this, but it does not matter if you are certain that you won't be involved in a war soon - morale takes about 4 month to go back to 100%, so this is how long you have to plan in advance. Consider that after a war is finished, it is unlikely for anyone to start one again soon because it would break the peace treaty. Also remember that you can delay accepting an ally's call for war assistance for (I think) up to 2 month.
Mothballing your ships
While you are at peace, you should mothball the ships you won't need, namely your galleys and your heavy ships, and even your transports if you are not planning to move your troops around. Only your trade ships should be fully funded to protect trade and/or privateer. Depending on your fleet, this trick can save you a lot of ducats.
Loans are a two edged sword. You'll have a large advantage in fights (due to more troops/mercs) and especially early fights, but unless you get good compensation from the war (looting and peace payments) you'll be in trouble afterwards. Worst case, you'll even end up with a disaster.
Remember the AP hit they imply, too, for reducing inflation in the future. So it may be better to simply lose a war than take out too many loans.
Obviously, you absolutely must get rid of all mercenaries in times of peace rather immediately. The math is easy to calculate how long your expected peace period needs to be to discard them (if unsure, just get rid of them and hope for the best).
You also need to get rid of costly advisors. Early game, it's not unusual to have none at all, and later, you'll want to use the cheap ones.
Value of buildings
Buildings are net positive only in the long run, so getting them early seems sensible. However, also consider that buildings in early times have a lot less value than in late times while they always cost the same to build.
For instance, influencing trade is a good thing, but if the trade value in your node is only 2 gold of which you now get 0.8 instead of 0.6, it will take awfully long until you get back the 50 (I forgot) gold a building costs to erect. Later in the game, the same node may hold a trade value of 15, and the calculation changes completely. Same thing with taxes.
So if you were to build something for its minimal gains, you would lose a lot of money you could spend on high impact factors when needed, for instance mercs or a larger army in general. Consider that this may leads to you winning a war more easily, so you get can claim more monetary compensation from the enemy than you could without the better military.
Even if it's just 10 gold you gain this way, it's likely a lot more than what early building gave you. Not to mention a lot more secure, too - having money makes you more flexible.
Applicability of this answer
It goes without saying that this answer does not apply to anyone attempting the Three Mountains or similar, but if you're a beginner and just having fun, I believe this should help you get a good start.