Try to vividly imagine the sensation of 'real' sex just before the act, then once you are near enough that imagination will morph into the real (lucid) thing. I hope that made sense haha.

Basically, i find thinking things like "i know i can do xyz because im lucid" doesnt really help, you have to affirm it eg "I am doing xyz" and try to counjure all the relational sensory attributes of what you want to acheive in your mind (whilst lucid dreaming of course) this requires a high level of lucidity, which can be difficult to acheive when on a hormone fuelled quest for sex, but definitely achievable!