I long ago decided that the Compose key is the best solution to this problem. In many cases, the Compose sequence is intuitive, e.g., Compose+l+- gives you £, C

I long ago decided that the Compose key is the best solution to this problem. In many cases, the Compose sequence is intuitive, e.g., Compose+l+- gives you £, Compose+t+m gives you ™, Compose+*+l gives you λ, and Compose+e+' gives you é. Compose is easy on Linux/BSD (some variation of setxkbmap will generally let you remap a key as Compose), and there's a WinCompose for Windows which behave essentially identically. There's a sort-of-solution for MacOS using Karabiner, but I've forgotten the details. The thing about Compose is that it fits in with touch-typing, whereas choosing the character from a menu means you have to stop, look at the menu, and select the desired character, before going back into typing mode. Perhaps that's good for people who almost never use non-ASCII, but that would definitely slow me down.