Formal:
* Chère Christine: This is the most formal option. It's appropriate for someone you don't know well or someone older than you.
* Madame Christine: This is even more formal and can be used if you're writing a letter to someone with a title, such as a business contact.
Informal:
* Chérie Christine: This is a more affectionate option, appropriate for a close friend or family member.
* Christine, ma chère: This is a very friendly and warm greeting.
You can also just use "Christine" directly, which is considered acceptable in many situations.
Ultimately, the best way to address Christine in French depends on your relationship with her and the context of the situation.