There is a fertilizer specially formulated to keep your hydrangea pink. You apply once a year in the spring and it will keep the plant healthy and in color. You should be able to find it at your local garden center. If you can't find it, adding dolomitic lime to the soil helps keep the plant pink by raising the pH level and, therefore, keeping the plant from absorbing the natural aluminum that's in the soil. The aluminum is what turns flowers blue.

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 Also, a fertilizer ratio of 10/30/10 is good for hydrangeas, but if you can search for the pre-packaged stuff specially made for pink hydrangeas, it makes the whole project a lot easier. The product is a slow-release granular fertilizer and it comes in a pink plastic container (there's a blue one for blue hydrangeas as well).