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Crikey! Don't hold your Xacto knife like you're going to stab the board! Use your Xacto knife with care. Always make sure your blades are sharp, and tape or post your board or paper so your hand (right if you're left-handed and visa-versa) is not under the blade. Always use a METAL ruler with a rubber backing when cutting edges to help keep your hand and the blade from slipping.
This may sound simple, but I'll guarantee there's been more blood shed on drafting tables than all the World Wars.