Hi Denise,
You didn't mention what type of metal you are using. Sterling, copper, brass and bronze will all tarnish naturally. how quickly depends on the climate you live in (more humid climates tarnish more quickly) and your natural skin chemistry. You can clean all of those beads regularly and remove the tarnish, using various methods.
If you are using base metal with a silver, copper, gold or brass plating, the plating will eventually wear off, leaving the dull base metal showing through. There's really nothing you can do about that. You can try fixatives, but its not a great long term solution.
There have been several threads in the past about cleaning tarnished beads, etc. If you do a search, you can read through them and probably find more info for your specific problem.
Erin