Wet your hair down with tomato juice and let it sit for a minute, then shampoo and condition as usual (though you might want to lather up one extra time for good measure.) For a stronger treatment, you can also smear tomato paste all over your hair, let it sit for 5 minutes, then shampoo.
Just don't make the mistake I did and substitute V8 Juice for tomato juice. Because it is tomato-based, I thought it would be fine. It worked on the chlorine, sure, but when all was said and done and the juice was shampooed out, my hair was still full of vegetable pulp. Ewww, it took several more shampoos to get it all out. Remember: just juice or paste, no pulp!