|("Paaie" by Handmaden Designs LLC)|
Now, if you work in wire (with no soldering) you can get wire that is pre-coated with a non-tarnish finish, but if you work with sheet, soldered wire-work, or want to retain some of the tarnished look without the issue of skin turning green who will need to coat the copper yourself.
How? Well, there are actually many different options. If you have the extra money to spend, you could go with Renaissance Wax. This is a common sealant used on jewelry to prevent tarnish, or the wearing off of patinas. Unfortunately, Renaissance Wax tends to be a bit on the pricey side. It is, however, not the only option available. I've heard of people using clear nail varnish to coat metal. While this will work for a while, it does tend to chip and eventually wear off.
I've been experimenting with this issue, and so far I have had great success with using a Fixative spray. Most often, Fixative is what artist's use to prevent their drawings from wearing off and fading (particularly charcoal or pastels). However, it can also be used on other surfaces.
I've used in an multiple pieces, and it's worked very well. On some pieces, I even left some of the tarnish on so I'd have the gorgeous affect of both the darkened copper and areas of bright copper on the same piece. I coated those pieces months ago, and they look just the same now.
Their are other products out there that are similar to Fixative that would likely work as well, this is just the one I had on hand and it has, thus far, been a very good inexpensive, alternative to Renaissance Wax. When using Fixative make sure to have a great deal of ventilation or wear some form of a respirator/mask (I used it outside, can't get better ventilation than that), as with any similar product, you do not want to breath in its fumes.
Spray the copper after all forming or soldering has taken place, but before adding any stones or beads. I would not recommend it be used on anything with moving parts, unless they are all sprayed separately and assembled afterwards. Also, it cannot be used with chainmaille (for the same reason as any articulated piece) as the fixative will cause it to stiffen, losing all movement and ability to bend. So with chainmaille, either be prepared to keep the tarnish, or use pre-coated wire from suppliers like Parawire.