I keep a small sized notebook with the first page or so an index of sort, and then a page for each service or server. Doesn't have to be a lot, but be sure to give your future self all the info that will be needed. For instance not just a password, but also the username. Any problems you have had and how you resolved it. Depending on the sophistication of your network, vlans, firewall info, is it wired to your router or switch, what port. What slot of your multi button power bar it is plugged into, so if you need to cut power or restart you dont have e to randomly push switch accidently turning something else off. Basic server specs, what type/size of raid or HDs, do you have room for more HDs later, RAM slots and what's in them currently.

I might get some flack from writing down passwords, but a password manager can remedy that. I still keep some of mine on paper tbh, I have had pw managers break or go offline and I am not terribly worried about normal theifs knowing how to ssh into my linux boxes.