Man, this used to happen to me SO much. I hated it, and it really made me kind of give up on lucids for a while. I only made it out of bed once while I was having the FA. (Every other time I would only be able to lift my arm, but my body was really stuck to the bed, no matter how much I told myself it was a dream)
The one time I made it, I had seen a pair of my boxers on the floor, and I told myself, just focus on picking up those boxers and I will forget about being stuck. I did it and it worked. But other times I haven't really been able to see something and focus on it.

I assume this will still happen to me, but I really haven't been focussing at all anymore on achieving it.