My son used to have them. I did lots of research online and did a couple things. First, when he had one, I didn't touch him. I sat by his bed and said reassuring things like, "It's OK, don't worry, you're going to be fine, Mama's here", etc. When I would touch him, he would freak out more. I think that by touching him, I became part of the terror... like I was the bad guy trying to hold him down and torture him or something. So no touching until they calm down and you can see them kind of "snap out of it". Once he snapped back, I would rock him and sing, etc until he fell asleep.
Second thing I did, which was the best thing I ever did, was get rid of all his pj's with feet. It sounds weird, but I read somewhere online that being overly warm can trigger night terrors. So I ditched all the foot jammas and he sleeps in lightweight pjs year round. In the winter I'll give him a moderate blanket, so that if he gets cold, he has it, and if he gets hot, he'll kick it off. Anyway, since I did that, he hasn't had a SINGLE night terror. It's been probably 6-8 months since his last one.