Formal:
* Tu es constamment dans mes pensées. (You are constantly in my thoughts.)
* Je pense à toi en permanence. (I think about you permanently.)
Informal:
* Je pense toujours à toi. (I always think about you.)
* Tu me traverses l'esprit tout le temps. (You cross my mind all the time.)
* Tu es dans ma tête. (You're in my head.) - This is a more informal and playful way to say it.
Romantic:
* Tu es mon obsession. (You are my obsession.)
* Je ne peux pas t'oublier. (I can't forget you.)
The best choice depends on the context and your relationship with the person you're talking to.