This is a huge wall of text, but I'm bored and like talking about this stuff.
Preserving retro consoles can be a pretty large rabbit hole. Over time, things are more likely to break and stop working. Storing your console in a temperature controlled environment with low humidity and away from direct sunlight will prevent your system from wearing down as quickly. Some basic soldering skills and electronics knowledge can help you diagnose and repair problems if they come up. You might not necessarily want to invest in these skills if all you care about is your one N64, but you will probably need to pay someone down the line to repair it when faults develop.
You should get comfortable with opening up your N64 and inspecting it for any signs of damage or corrosion. If there's anything that looks like it's bulging like electrolytic capacitors, or if something looks corroded, those will require attention. If the metal shielding has spots like that, it's not super important, but corrosion on the motherboard can cause things to fail. If things look dirty, use some isopropyl alcohol and q-tips or other gentle cleaning implement to get rid of any buildup you see. I would suggest getting 99% isopropyl alcohol to clean your electronics. It cleans better, and it evaporates quickly. If you turn on your console while it is still wet from cleaning, you can cause a short ciruit and potentially fry your console. There may also be bugs alive or dead that have made homes inside your console which will need to be removed. Make sure you use something non-conductive to clean. Don't use like a bath towel or something. Things like that can build up static electricity which can discharge into the various electronics in your system causing them to fail. Also try to make sure you're not going to build up any static charge (don't work on your console in a carpeted room for example). There are grounding wristbands you can wear to ensure you don't build up static, though it's not a strict requirement to wear one. There's probably going to be some brown gunk on the motherboard. This is soldering flux from production. You can clean it off if you want, but it's not essential. It can be quite sticky and stubborn to remove. Also make sure you open up and inspect/clean your power supply and controllers.
The joysticks are also a concern. There is a finite supply of OEM joysticks, and they wear down over time with use more than other analogue sticks. Proper cleaning and lubrication of the parts can increase the life of the sticks. Modern replacements tend to not feel the same, and some games are designed very specifically for original sticks. A gamecube stick will definitely not feel right for some games due to the lack of rigidity and different range of motion. Your mileage will vary depending on what games you play. Original sticks still feel the best imo. There is a metal replacement for some components of the joystick, but it's not a full parts replacement, and they're also stupid expensive.
Cartridge contact corrosion is also a concern for long term preservation especially when most consoles have had moisture introduced through blowing on cartridges. Using some contact cleaner on the cartridges/slots can lubricate the pins on the slot and help remove/prevent buildup. Lubrication will also prevent wear from inserting and removing cartridges. Use some alcohol to clean the contacts of your cartridges before doing this. Avoid using sandpaper on your cartridge slot, as it can remove the plating on your cartridge slot's pins. If there seems to be a lot of buildup in your catridge slot, you can apply some contact cleaner to a cartridge and repeatedly insert and remove it from the system. Do it with a cartridge you don't care about as much because it will add some wear to the contacts. Dirt buildup in the expansion/booster pack slot can also cause the console to reset randomly.
Another consideration is save batteries. The batteries will eventually die in cartridges, and you will be unable to save until you replace the battery. The controller pak also has a save battery in it that will need replacing. This requires some basic soldering proficiency. People sell battery holders you can install so you don't have to desolder the old battery when it dies. If you care about the saves on them, you'll need to dump them with some sort of dumper device before replacing the battery. These devices should be capable of writing the save back to the cartridge as well. Personally I haven't fiddled with dumping, and am not super attached to my old saves. If you own a rumble pak, you should also be sure to remove the AAA battery from it if you don't plan on using it for a while. The battery can leak and potentially ruin your rumble pak.
You may also want to consider getting the electrolytic capacitors replaced at some point. I don't think this is super urgent, but they can be a point of failure for old electronics. Electrolytic capacitors are filled with conductive fluids that eventually can dry up over time and lose their ability to store an electric charge. Surface mounted capacitors like the ones on the N64 motherboard can be problematic because they are more likely to leak fluid onto the board which can lead to corrosion. I haven't heard of any caps failing on N64's, but it is something that will happen eventually. The generally stated shelf life of these capacitors is said to be 20-30 years. There are capacitors in older electronics that still work fine, and there are some in newer ones that have already failed. Google original xbox clock capacitor leak to see examples of what bad electrolytic capacitors can do. Leaking capacitors don't damage your system right away, but if you're going to be putting your console away for several years and a capacitor leaks, you might take it out and find out that your console no longer works due to corrosion of the copper traces on the board. I've been recapping my old consoles just as preventative maintenance. It requires a bit more skill than battery replacement to change capacitors. If you improperly remove the old capacitors or make a mistake when placing the new ones, you can damage your system. It's also very time consuming and tedious to replace capacitors. There are people who offer recapping services if you would rather have someone else do it.
This may sound like a lot of effort, and it is. I find it really fun to work with and collect old systems. It also gives me more appreciation for things like software emulation and FPGA that seek to preserve the experience of old games. It's uncharted territory as far as how long these old systems will last. My main concern is that proprietary chips like the CPU will fail eventually, and there aren't going to be replacements available. I hope you get many years out of your N64. Cherish what you have while you can still enjoy it.
