Just hack them back as soon as they have finished flowering - seriously. That will give them time to set more buds for next year's flowers. I have three evergreens and one deciduous, and it works for each of them.
Grit - well, better drainage should help. As Jess says, they come from well-drained hillsides in China, with shade provided by mountain slopes and trees, as do many clematises. They often have little water, but well-mulched/shaded roots. Mine are well-drained and grow extremely fast. Three of them are growing under trees, one between a small tree and a garden wall, so don't get much chance to be water-logged. All you can do is experiment or move the plant, after cutting it back by about a third.
Yes, do please have a look Jess, because I haven't had any rain for weeks until last night, but I live in a frostpocket in east Anglia, so often end up with browned edges. Rain doesn't seem to affect the varieties I have, but maybe it does others. A bit like roses.