I first read this in Witchcraft For Tomorrow by Doreen Valiente.  It's yet another wonderful piece the can add real magick to a rite (though I often change one repeated part to "So Mote it Be!").  Enjoy!

The Coven Spell
by Doreen Valiente

O ancient ones of heaven, earth and sea,
We chant the coven spell, thus shall it be!
To music of the night-wind blowing free,
We chant the coven spell, thus shall it be!

The owl hoots within the hollow tree,
The black cat runs by night silently,
The toad beneath the stone dwells secretly,
We chant the coven spell, thus shall it be!

To moon that draws the tides of air and sea,
We chant the coven spell, thus shall it be!
To god that bides beneath the greenwood tree,
We chant the coven spell, thus shall it be!

By witches' garter bound about the knee,
By staff and cauldron and all powers that be,
We will the thing in our minds we see,
We chant the coven spell, thus shall it be!

(Pause...)

The Spell is flowing like the sea,
The spell is growing like the tree,
Like flame that burns and blazes free.
We chant the spell, thus shall it be!
We chant the spell, thus shall it be!
We chant the spell, thus shall it be!
IT IS!