Cast circle about yourself and call the four watchtowers. Sit in front of your altar with the athame in the center of your working space. Calm and center yourself, reach out your mind to the Lord and Lady. When you are ready, focus on your athame and say:
I cleanse and purify this tool of my Craft from all past energies and negativity.
Visualize all the past associations of your knife and all negative energies shooting forth, leaving the knife and your circle.
Spirits of Earth, bless this athame by the steel of it's blade, to lend strength and stability to my magick and work
Sprinkle some salt onto the blade and handle of your athame
Spirits of Air, bless this knife with the power to direct my magick on the journey I send it, as it alone may penetrate through space and time.
Cleanse your athame.
Spirits of Fire, who forged this blade, bless my athame to be the instrument of my Will, free of negative influences and pure in it's direction of my energies.
Run the blade through a candle flame.
Spirits of Water, bless this magickal knife to be a tool used with love, and in respect for the mysteries. May all spells it casts be within the natural rules of using the Power.
Sprinkle water over the athame
Now, to the Goddess and God, say:
I bless this athame in the names of my Goddess and God that it may be true to the Will of my spirit, to be a powerful tool of the Lord tempered with the intuition of the Lady.
In love and in trust,
In peace and in wisdom,
I bless this tool of my faith,
So mote it be.
Taken from: http://everythingunderthemoon.net/forum/athame-t20296.html