'Knife project' works well here :

• Add a plane
• Place one of its edges along the middle (at the exact center)
• You may work in a ortho view and along the axis (here top ortho)
• Stretch it so that it covers your mesh
• Select your mesh (keeping the plane selected) and enter edit mode
• Hit 'knife project' and make sure the option is 'cut through'
• Select the half to remove and add the mirror modifier

http://blend-exchange.giantcowfilms.com/embedImage.png?bid=1735

'Knife project' works well here :

• Add a plane
• Place one of its edges along the middle (at the exact center)
• You may work in a ortho view and along the axis (here top ortho)
• Stretch it so that it covers your mesh
• Select your mesh (keeping the plane selected) and enter edit mode
• Hit 'knife project' and make sure the option is 'cut through'
• Select the half to remove and add the mirror modifier