Earlier acceptance of the germ theory would also be important - or is that part of that TL; I've never read it.
Smallpox eradicated 100 years before it was, same with many other diseases, would really help a lot. Not sure how that happens with a post-1800 POD, someone's trying it in the SHWI-ISOT game (just search for SHWI-ISOT) over in yahoo groups, but it's a tough go convincing people. You'd need someone with a very determined nature.
avoid World War One or make it a very quick win, and that helps, too. But, you're right, it would have to be something revolutionary that really pushes it over the top.