"I am praying for you"
Rezar and orar are NOT the same. Rezar is when one recites a prayer - much as when one prays to Mary the Holy of God in the Roman Catholic Church (or the traditional "now I lay me down to sleep" that some of us learned as children). Orar is more of a conversational prayer - much as those spoken to God in the Christian church, whether by clergy or laymen. You can see this difference between the definitions from Real Academia Español in Issabela's answer.
Thus, if you're really praying for someone in the sense that, "I'm talking to God for you", then the answer is, "Estoy orando por ti".
Since Issa's answer, now we have the question. What is the difference between "rezar" and "orar"? Just for fun
1. tr. Dirigir a Dios o a personas santas oraciones de contenido religioso.
2. tr. Dicho del clérigo obligado a ello: Recitar el oficio divino vocal u oralmente.
3. tr. Rel. Recitar la misa, una oración, etc., en contraposición a cantarla.
1. intr. Hacer oración a Dios, vocal o mentalmente.
2. intr. Hablar en público para persuadir y convencer a los oyentes o mover su ánimo.
3. tr. Rogar, pedir, suplicar.
I think we need some more explanation, because as far as I understand, they're just synonyms.
No difference, regional usage and may be more used rezar.
Rezo por ti.
Todos rezamos por Alberto y su familia
I use the verb "rogar" while praying in church. I use it because it is found most often in my version of the Bible when Jesus is praying.
It carries a sense of urging, pleading, begging.
So I guess my answer is "Estoy orando por ti", but, if directing a prayer, I use the conjugated form of "rogar".
That probably make no sense to anyone except me.
I thought that I would look up the verb used to indicate what Jesus said when he was responding to his disciples request for him to teach them how to pray in Luke chapter 11
"Y les dijo: cuando oreís, decid: padre nuestro que estás en los cielos,sanctificado sea su nombre.....''.
Also in Matthew ch 6 v 5:
''Y cuando ores, no seas como los hipócritas, etc...."'
I have to say that I am in complete agreement with on mission. I asked the same question once to our Youth pastor, from Brazil, when he was teaching me Spanish.about 3 years ago (He had lived for 5 years while growing up with his family in Mexico) He told me the same thing that the catholics use the verb rogar to translate the recited prayers they use in church whereas evangelical christians use the verb orar which suggests a more conversational type of prayer to God.
I hope this helps