I ride a 1989 Cannondale, and until last year have always had the same rear cassette, and maybe changed the chain once. Last year I changed both. Before that I have NEVER had any rust, but now I've noticed some rusting on both. Can someone tell me what went wrong, and more importantly how to remedy the situation?

Rust is caused by water exposure and lack of protective lubrication. You also changed one or both when you changed the cassette and chain.

To add to the above. Left alone, the drivetrain accumulates a film of waxy/dirty oil residue, which is an effective rust preventative. New stuff and stuff that people religiously keep clean lacks this nasty film and is more pone to rust.

In any case, minor surface rust isn't a crisis, and is rapidly burnished off the working areas the same way trains keep the tracks polished. However don't let rust get deep on the surfaces of the chain plates as they're thin to begin with, and don't have much reserve strength. (minor rust around the plates' rims is OK).

Your original equipment have have been higher quality, plated (nickel?) than the the replacements. The prevention is stated above. Reduce exposure and lubricate.