You could say:
"I have you on my mind." This has a positive meaning, that you are thinking of the person fondly.
You could also say:
"I can't get you out of my mind." This means you are obsessing about the person, thinking of them all time.
or
"I can't get you out of my head." (But you wouldn't say "I have you in my head")
This has a slightly negative meaning, that you are thinking of the person when you don't want to be thinking of him or her. You could also say "I can't get this song out of my head!" when it is bothering you.