Formal:
* J'attends avec impatience votre réponse. (This is the most literal translation and the most formal.)
* Je vous remercie de bien vouloir me répondre. (This is more polite and conveys a sense of waiting for your response.)
Informal:
* J'attends de vos nouvelles. (This literally means "I await your news" and is a more casual way to say "I look forward to your reply.")
* J'espère avoir de vos nouvelles bientôt. (This translates to "I hope to hear from you soon.")
Other options:
* Je vous remercie d'avance pour votre réponse. (This is a polite way to say "Thank you in advance for your reply.")
The best way to choose the right phrase depends on who you are writing to and the context of your message.