Water pump flange o-ring leaks

I recently did a timing belt replacement, and did the water pump at the
same time. Everything worked fine, but after about 10 miles, the water
pump flange o-ring started leaking like crazy.

I thought it was on ok when I pushed the flange on, but now I can see
it is hanging out of the groove a bit. Now I have to tear down the
whole thing and start over.

Anybody know how I can prevent this from re-occuring? Do I need to
lubricate it before pushing it on.????