Heartrate, and wakefulness.
Decrease the amount of time AND ACTIVITY between waking up, and going back to sleep. It has actually helped me to wake up, and read the DV forums, and then lay on the couch. It does seem to help me a lot if I attempt my lucids in a separate place from where I usually sleep. Sleeping on the couch helps remind me that "this is lucid dream time". If you are having a hard time falling back asleep, try not to move around/walk around as much. Maybe just crawl to the couch. lol
It should be noted: In my personal experience, this greatly increases my chances of experiencing sleep paralysis, but it also greatly increases my chances of hearing the "buzzing" or "vibrations" as others have described it. To me, it sounds like static, and it does NOT lead to a lucid dream for me. It leads to awful electrical shocks shooting through my body, and sometimes sleep paralysis. However, it has long since been clear to me that this is not the case for most people. For most people this causes lucid dreams.
I have also had decent success initializing a WILD in a SIMILAR way as WBTB by just completely wearing myself out, by doing yard work in the summer, then laying on the couch.