Formal:
* Jusqu'à notre prochaine rencontre, je penserai à vous. (This is the most literal translation and is suitable for a formal context.)
* En attendant de vous revoir, je garderai votre souvenir. (This translates to "Until we see each other again, I will keep your memory." It's a slightly more poetic option.)
Informal:
* À bientôt, je penserai à toi. (This is a more casual way of saying it, using "à bientôt" which means "see you soon.")
* J'ai hâte de te revoir, et je penserai à toi d'ici là. (This translates to "I can't wait to see you again, and I will be thinking of you until then.")
Romantic:
* J'attends avec impatience notre prochaine rencontre, et je ne t'oublierai pas d'ici là. (This translates to "I eagerly await our next meeting, and I won't forget you until then.")
You can also add a phrase like "avec affection" (with affection) or "avec tendresse" (with tenderness) at the end to add a touch of warmth.
The best choice will depend on the context of your situation and your relationship with the person you are speaking to.