Wash your locks with a clarifying shampoo to get your strands cleaner than clean. A clarifying shampoo will help remove buildup from styling gunk, allowing the tomato juice direct access to your follicles. You don't have to do this, but it'll help.
Put a large bowl in your bathroom sink, kitchen sink or tub. Lay out everything you'll need -- cup, plastic wrap, shower cap and tissues to wipe up drips -- within arms reach. This trick can get messy, and you don't want to run dripping tomato juice through the house to grab a shower cap at the last minute. Once you've set up your tomato juice station, crack open your tomato juice and hang your head over the bowl. Then just dump the juice on your hair.
Massage your tomato-slathered locks a little to get the juice in all your mane's nooks and crannies, keeping your head over the bowl to collect drips. Then, dip your cup in the bowl to collect another round of juice and give your locks another splash of juice. Keep on dunking the cup in and pouring more juice on your locks -- the more, the better. Do this about 15 times to make sure your locks are sufficiently tomatofied.
Lightly squeeze out excess juice, then pile your hair on top of your head and wrap it up in plastic wrap. After you're all sealed in plastic, throw a shower cap on for extra drip protection.
Give the tomato juice and your hair time to get to know each other. Try to hang out for at least 15 minutes or more. Keep some tissues by your side to wipe up any juice that escapes from your plastic wrapped fortress. If it gets too drippy, wrap up in an old towel. When you're ready, rinse your strands without shampooing.
You probably won't see crazy results the first time, but keep on doing this trick about once a week and you'll start to see a progressive change.
Again, this can get messy. Wear old clothes and stay far away from carpet.