You won't necessarily be able to prevent tarnish from forming, but there are a few ways you can significantly delay the formation of tarnish.
The best way is to wear your sterling silver jewelry frequently. The oils in your skin will "clean" the sterling silver every time you wear a piece.
The second best way to delay tarnishing is to store sterling silver in an airtight container, preferably with an anti-tarnish strip (a specially formulated material that absorbs moisture). Anti-tarnish strips are available at Aquablue and are included with every sterling silver jewelry purchase.
While it may not look as pretty, plastic zip-top bags are the best container to use to store your jewelry. A jewelry box with a tight-fitting lid is also a good option.
I store my jewelry in a kraft jewelry box (like your jewelry purchase from Aquablue comes in) and wear the jewelry frequently. I have not had any of my personal jewelry, which I've had for up to a year, tarnish at all. In fact, it is incredibly shiny with this limited very limited amount of care.