In a recent article by Bethany Lyttle she gives her advice on how to dry Hydrangeas:

• “The best type of Peegee hydrangea is the standard tree form, not the bush,” she explains. “Longer branches mean blooms. In French Quebec, the Peegee variety is know as Quatre Saisons (Four Seasons), because the flower goes through four stages of color — white to green to pink and to brown.”
• “You know they’re ready to pick for drying when a burgundy tone forms on the tips and the flowers on the branch overall are firm to the touch.”
• “I look for only those hydrangeas that have a defined conical shape. It’s an appealing look and they hold that shape.”
• “Because of their pronounced weight, I hang them upside down to dry. The green and burgundy tones stay vibrant for up to a year. Beyond that, they then turn an appealing beige color.”