I know this is an older thread, but i figured i would reply, as i have been practicing a few takedowns lately, from a grappling only perspective.
I like the high double leg that BJ Penn shows in his MMA book. Its easier for me to get more lifting power from there than from a regular low double, though that could be lack of technique on my part.
At my school we also do it where the head rams the center of the chest as you grab behind their knees and rip their legs out from under them. I think they call it the blast double. I know it sounds kind of unsophisticated, but it works. You just need to watch the guillotine if you end up in their guard.
I think the key to all takedowns is the setup, which i am bad it. For those above i usually do them from collar/elbow clinch. A snapdown, elbow pop, or head pop all seem to work. You just gotta be fast and commit.
Nice blast double here on Coach Rut's blog.