My biggest piece of advice is get small goldfish; the younger they are when they get used to plants the less likely they will eat them when they're older. Also, sometimes you just have to get lucky with the temperament of each fish. Making sure that they get enough plant matter can help too; mine's diet is about half algae wafers.
My goldfish doesn't really eat a lot of my plants, just pulls them up until they have good roots, or knocks them out of the ground by nosing around the substrate (I have a comet).
Plants he eats are: duckweed, frogbit (pretty much any floaters get eaten or shredded), and he trims the java moss so that it stays close to the log (which I actually like). As a previous poster mentioned, I use him to git rid of extra floating plants from my shrimp tank.
Don't count on any carpet plant unless you can get it established first; goldfish are just too curious and will pull it out, causing you to pull your hair out. You might try a bigger carpet like dwarf sag which tends to have a larger root system to plant, but be prepared to keep reinserting floaters.
Plants that he doesn't eat in my low tech set up are crypts, several shapes of java ferns (narrow leaf ones do fine too), pennywort, Hygrophila polysperma, H. difformis, golden creeping jenny, swords, and hornwort. I've heard people say that they eat hornwort, but he totally ignores it.
Plants I haven't tried but have heard will work are crinum sp. and various lotuses.