Don't be deterred by the sinking feeling you get if you have the 'calm before the storm' reaction, either.
DD3 had a nightmare a couple of weeks ago. We soothed her. Then she decided she'd like to sleep in our bed. We said no.
She proceeded to start screaming and shouting that she wanted to sleep in our bed. I said 'no, DD3, that's not happening, but I will go and get my pillows and sleep on your floor.' That wasn't good enough for DD3, because we were at different heights (she sleeps on the top bunk). I did my calm parenting bit, and then after some time said 'let me give you a hug, you must be so tired.'
DD3, miraculously, seemed calm and sanguine again. She politely told me that she nolonger needed a towel on her pillows (she'd gone to bed with wet hair) and asked me to remove it. Then she said 'I do want to take these with me.'
I said 'Take them where?'
DD3: Your bed 
My heart sank because I realised that she'd calmed down because she thought I'd given in.
So the whole thing started again. Eventually, though, she did come to the end of it because she realised that I would stand there all night rather than let her sleep in our bed.