Storing your brass in an airtight container will slow the process of oxidizing, but it will still tarnish even when stored well.
I use the 3m polishing papers to clean my brass. I have found that using fine sandpaper gives the wire a slightly matte sheen and I prefer the glossy look that the polishing papers give the wire. You can use up old pieces of polishing paper, they still remove the tarnish.
Once you have made your jewelry you will either need to constantly re-polish it or coat it with something to block the air and moisture. I use a natural beeswax lip balm to create a barrier to slow the tarnish. The natural beeswax lip balm is already made to a usable consistency and it shouldn't react even with sensitive skin. You don't need to smother the brass in wax, a little bit does go a long way.