Lemon juice with salt works wonders on copper. Some just dip it in and save the cup of it for when you want to use it again, I use a bit on a cloth and give it a rub.
I have used ren wax to hold a shine on wire. It needs to be reapplied periodically but it does work great and it is safe.
I have heard that fingernail polish will peel off in places and the piece will look horrible. I have no experience with that though.
I personally prefer copper to tarnish naturally. It shines up in places and gets an interesting patina. I like natural.
You can also patina your copper yourself. Gun blueing looks amazing. Sherry "Dancing Feather" uses that technique a lot. It gives the copper a beautiful patina and then the piece does not really change much in color.
All my brass pieces have stayed somewhat shiny, natural. I have never needed to clean them. I am not sure if lemon would work on brass or not.
Ketchup also cleans copper beautifully, so I hear :D (no really).
Szarka