Not everyone believes in worming, but this is what I was told to do as a preventative measure:
Worm when 6 months old, then every 6 months after a that, in the spring after the thaw and in the fall after the hard freeze.  If you have a very wet summer, you should worm in mid-summer as well.  A full clean-out of the coop should be done at this time as well.  Sunlight kills the eggs.  Note:  Earthworms are carriers of roundworms.
To worm:
1.  Take chickens off water starting the evening before you plan to worm.
2.  Mix Piperazine 17 (aka Wazine 17) 1 oz per gallon of water.  Give to chickens (NOT for baby chicks.)
     Other wormers (if you can't find either of the above):  Amprolium, Panacur
3.  The next day, give them regular water.
4.  Repeat worming in 14-21 days.
5.  Do not use the eggs for 5-7 days after worming.
I have also read that raw pumpkin, or coal dust are natural dewormers.